ccREL: The Creative Commons Rights Expression Language
Hal Abelson, Ben Adida, Mike Linksvayer, Nathan Yergler[hal,ben,ml,nathan]@creativecommons.org
Version 1.0 – March 3rd, 2008

1

Introduction

This paper introduces the Creative Commons Rights Expression Language (ccREL), the standard
recommended by Creative Commons (CC) for machine-readable expression of copyright licensing
terms and related information.1 ccREL and its description in this paper supersede all previous
Creative Commons recommendations for expressing licensing metadata. Like CC’s previous recommendation, ccREL is based on the World-Wide Web Consortium’s Resource Description Framework (RDF).2 Compared to the previous recommendation, ccREL is intended to be both easierfor content creators and publishers to provide, and more convenient for user communities and toolbuilders to consume, extend, and redistribute.3
Formally, ccREL is specified in an abstract syntax-free way, as an extensible set of properties tobe associated with a licensed documents. Publishers have wide discretion in their choice of syntax,
so long as the process for extracting the properties is discoverable and tool builders can retrieve theproperties of ccREL-compliant Web pages or embedded documents. We also recommend specificconcrete “default” syntaxes and embedding schemes for content creators and publishers who want
to use CC licenses without needing to be concerned about extraction mechanisms. The default
schemes are RDFa for HTML Web pages and resources referenced therein, and XMP for standalone media.4
An Example. Using this new recommendation, an author can express Creative Commons structured data in an HTML page using the following simple markup:
1
Information about Creative Commons is available on the web at http://creativecommons.org. ccREL is a
registered trademark of Creative Commons, see http://creativecommons.org/policies for details.
2
RDF is a language for representing information about resources in the World Wide Web. We provide a shortprimer in this paper. Also, see the Web Consortium’s RDF Web site at http://www.w3.org/RDF/.
3
By “publisher” we mean anyone who places CC-licensed material on the Internet. By “tool builders” we mean
people who write applications that are aware of the license information. Example tools might be search programs thatfilter their results based on specific types of licenses, or user interfaces that display license information in particularways.
4
RDFa is an emerging collection of attributes and processing rules for extending XHTML to support RDF.
See the W3C Working Draft “RDFa in XHTML: Syntax and Processing” at http://www.w3.org/TR/rdfa-syntax.
The “RDFa Primer: Embedding Structured Data in Web Pages,” may be found at http://www.w3.org/TR/
xhtml-rdfa-primer. RDF/XML, described briefly below, is a method for expressing RDF in XML syntax. See“RDF/XML Syntax Specification (Revised),” W3C Recommendation 10 February 2004 at http://www.w3.org/TR/
rdf-syntax-grammar/. XMP (Extended Metadata Platform) is a labeling technology developed by Adobe, for embedding constrained RDF/XML within documents. See http://www.adobe.com/products/xmp/.

This work is licensed under a Creative Commons Attribution License, v3.0. The license isavailable at http://creativecommons.org/licenses/by/3.0/. Please provide attribution toCreative Commons and the URL http://creativecommons.org/projects/ccREL.

<div about="http://lessig.org/blog/" xmlns:cc="http://creativecommons.org/ns#">
This page, by<a property="cc:attributionName"
rel="cc:attributionURL" href="http://lessig.org/">
Lawrence Lessig</a>,is licensed under a<a rel="license" href="http://creativecommons.org/licenses/by/3.0/">
Creative Commons Attribution License</a>.
</div>

From this markup, tools can easily and reliably determine that http://lessig.org/blog/ is licensed under a CC Attribution License, v3.0, where attribution should be given to “Lawrence
Lessig” at the URL http://lessig.org/.
Structure of this Paper. This paper explains the design rationale for these recommendations
and illustrates some specific applications we expect ccREL to support. We begin with a review ofthe original 2002 recommendation for Creative Commons metadata and we explain why, as Creative
Commons has grown, we have come to regard this as inadequate. We then introduce ccREL in
the syntax-free model: as a vocabulary of properties. Next, we describe the recommended concrete
syntaxes. In addition, we explain how other frameworks, such as microformats, can be made ccREL
compliant. Finally, we discuss specific use cases and the types of tools we hope to see built to takeadvantage of ccREL.

2

Background on Creative Commons recommendations

Creative Commons was publicly launched in December 2002, but its genesis traces to summer 2000
and discussions about how to promote a reasonable and flexible copyright regime for the Internet in
an environment where copyright had become unreasonable and inflexible. There was no standard
legal means for creators to grant limited rights to the public for online material, and obtaining
rights often required difficult searches to identify rights-holders and burdensome transaction costs
to negotiate permissions. As digital networks dramatically lowered other costs and engendered newopportunities for producing, consuming, and reusing content, the inflexibility and costs of licensing
became comparatively more onerous.
Over the following year, Creative Commons’ founders came to adopt a two-pronged responseto this challenge. One prong was legal and social: create widely applicable licenses that permit
sharing and reuse with conditions, clearly communicated in human-readable form. The other prong
called for leveraging digital networks themselves to make licensed works more reusable and easy tofind; that is, to lower search and transaction costs for works whose copyright holders have grantedsome rights to the public in advance. Core to this technical component is the ability for machines
to detect and interpret the licensing terms as automatically as possible. Simple programs should
thus be able to answer questions like:• Under what license has a copyright holder released her work, and what are the associatedpermissions and restrictions?
2• Can I redistribute this work for commercial purposes?
• Can I distribute a modified version of this work?
• How should I assign credit to the original author?
Equally important is constructing a robust user-machine bridge for publishing and detecting
structured licensing information on the Web, and stimulating the emergence of tools that lower thebarriers to collaboration and remixing. For example, if a Web page contains multiple images, not
all licensed identically, can users easily determine which rights are granted on a particular image?
Can they easily extract this image, create derivative works, and distribute them while assigningproper credit to the original author? In other words, is there a clear and usable connection between
what the user sees and what the machine parses? ccREL aims to be a standard that implementors
can follow in creating tools that make these operations simple.

2.1

Creative Commons and RDF

As early as fall 2001, Creative Commons had settled on the approach of creating machine-readable
licenses based on the World Wide Web Consortium’s then-emerging Resource Description Framework (RDF), part of the W3C Semantic Web Activity.5
The motivation for choosing RDF in 2001, and for continuing to use it now, is strongly connected to the Creative Commons vision: promoting scholarly and cultural progress by making it
easy for people to share their creations and to collaborate by building on each other’s work. In
order to lower barriers to collaboration, it is important that the machine expression of licensing
information and other metadata be interoperable. Interoperability here means not only that different programs can read particular metadata properties, but also that vocabularies—sets of related
properties—can evolve and be extended. This should be possible in such a way that innovation
can proceed in a distributed fashion in different communities—authors, musicians, photographers,
cinematographers, biologists, geologists, an so on—so that licensing terms can be devised by local
communities for types of works not yet envisioned. It is also important that potential extensions
be backward compatible: existing tools should not be disrupted when new properties are added.
If possible, existing tools should even be able to handle basic aspects of new properties. This isprecisely the kind of “interoperability of meaning” that RDF is designed to support.
2.1.1

RDF triples

RDF is a framework for describing entities on the Web. It provides exceptionally strong support
for interoperability and extensibility. All entities in RDF are named using a simple, distributed,
globally addressable scheme already well known to Web users: the URL, and its generalization the
URI.6
For example, Lawrence Lessig’s blog, a document identified by its URL http://lessig.org/
blog/, is licensed under the Creative Commons Attribution license. That license is also a docu5
The Semantic Web Activity is a large collaborative effort led by the W3C aimed at extending the Web to becomea universal medium for data exchange, for programs as well as people. See http://www.w3.org/2001/sw/.
6
The term URI (universal resource identifier) is a generalization of URL (universal resource locator). While a
URL refers in principle to a resource on the Web, a URI can designate anything named with this universal hierarchicalnaming scheme. This generality is used in ccREL for items such as downloaded media files.

3

ment, identified by its own URL http://creativecommons.org/licenses/by/3.0/. The property of “being licensed under”, which we’ll call “license” can itself be considered a Web object andidentified by a URL. This URL is http://www.w3.org/1999/xhtml/vocab#license, which refers
to a Web page that contains information describing the “license” property. This particular Web
page, maintained by the Web Consortium, is the reference document that describes the vocabulary
supported as part of the Web standard XHTML language.7
Instantiating properties as URLs enables anyone to use those properties to formulate descriptions, or to discover detailed information about an existing property by consulting the page at
the URL, or to make new properties available simply by publishing the URLs that describe those
properties.
As a case in point, Creative Commons originally defined its own “license” property, which it
published at http://creativecommons.org/ns#license,8 since no other group had defined in
RDF the concept of a copyright license. When the XHTML Working Group introduced its own
license property in 2005, we opted to start using their version, rather than maintain our own CCdependent notion of license. We were then able to declare that http://creativecommons.org/
ns#license is equivalent to the new property http://www.w3.org/1999/xhtml/vocab#license,
simply by updating the description at http://creativecommons.org/ns#license. Importantly,
RDF makes this equivalence interpretable by programs, not just humans, so that “old” RDF license
declarations can be automatically interpreted using the new vocabulary.
In general, atomic RDF descriptions are called triples. Each triple consists of a subject, aproperty, and a value for that property of the subject. The triple that describes the license for
Lessig’s blog could be represented graphically as shown in figure 1: a point (the subject) labeledwith the blog URL, a second point (the value) labeled with the license URL, and an arrow (theproperty) labeled with the URL that describes the meaning of the term “license”, running fromthe blog to the license. In general, an RDF model, as a collection of triples, can be visualized as agraph of relations among elements, where the edges and vertices are all labeled using URIs.
http://www.w3.org/1999/xhtml/vocab#license

http://lessig.org/blog/

http://creativecommons.org/licenses/by/3.0/

Figure 1: An RDF Triple represented as an edge between two nodes of a graph.
7
The vocabulary page, http://www.w3.org/1999/xhtml/vocab#, is currently a placeholder which the W3C expects to update in early 2008.
8
The full story is a little more complicated. CC initially used the http://web.resource.org/cc/ namespace,migrating to http://creativecommons.org/ns# for superior human interaction with the vocabulary when it became
apparent RDFa would facilitate this. In 2004 the Dublin Core Metadata Initiative approved a “license” refinement of
its “rights” term (see http://dublincore.org/usage/decisions/2004/2004-01.Rights-terms.shtml). Had http://purl.org/dc/terms/license existed in 2002, CC would not have defined http://web.resource.org/cc/license.
Thanks to the extensibility properties of RDF, http://creativecommons.org/ns#license describes its relationshipto each of these other properties.

4

2.1.2

Expressing RDF as text

Abstract RDF graphs can be expressed textually in various ways. One commonly used notation,
RDF/XML, uses XML syntax. In RDF/XML the triple describing the licensing of Lessig’s blog isdenoted:<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:xhtml="http://www.w3.org/1999/xhtml/vocab#"><rdf:Description rdf:about="http://www.lessig.org/blog/"><xhtml:license rdf:resource="http://creativecommons.org/licenses/by/3.0/" /></rdf:Description></rdf:RDF>

One desirable feature of RDF/XML notation is that it is completely self-contained: all identifiers
are fully qualified URLs. On the other hand, RDF/XML notation is extremely verbose, making it
cumbersome for people to read and write, especially if no shorthand conventions are used. Even thissimple example (verbose as it is) uses a shorthand mechanism: the second line of the description
beginning xmlns:xhtml defines “xhtml:” to be an abbreviation for http://www.w3.org/1999/
xhtml/vocab#, thus expressing the license property in its shorter form, xhtml:license, on the
fourth line.
Since the introduction of RDF, the Web Consortium has developed more compact alternativesyntaxes for RDF graphs. For example the N3 syntax would denote the above triple more concisely:9<http://lessig.org/blog/><http://www.w3.org/1999/xhtml#license><http://creativecommons.org/licenses/by/3.0/> .

We could also rewrite this using a shorthand as in the RDF/XHTML example above, defining:
xhtml: as an abbreviation for http://www.w3.org/1999/xhtml/vocab#:@prefix xhtml: <http://www.w3.org/1999/xhtml/vocab#><http://lessig.org/blog/>
xhtml:license<http://creativecommons.org/licenses/by/3.0/> .

The shorthand does not provide improved compactness or readability if a prefix is only used
once as above, of course. In N3, prefixes are typically defined only when they are used more thanonce, for example to express multiple properties taken from the same vocabulary. In RDF/XML,because of the stricter parsing rules of XML, there is a bit less flexibility: predicates can only be
expressed using the shorthand, while subjects can only be expressed using the full URI.

2.2

CC’s Previous Recommendation: RDF/XML in HTML Comments

With its first unveiling of machine-readable licenses in 2002, Creative Commons recommended thatpublishers use the RDF/XML syntax to express license properties. The CC web site included a
9N3 (Notation 3) was designed to be a compact and more readable alternative to RDF/XML. See http://www.
w3.org/DesignIssues/Notation3.html.

5

Web-based license generator, where publishers could answer a questionnaire to indicate what kindof license they wished, and the generator then provided RDF/XML text for them to include on
their Web pages, inside HTML comments: <!-- [RDF/XML HERE] -->.
We knew at the time that this was a cumbersome design, but there was little alternative.
RDF/XML, despite its verbosity, was the only standard syntax for expressing RDF. Worse, the Web
Consortium’s Semantic Web Activity was focused on providing organizations with ways to annotate
databases for integration into the Web, and it paid scant attention to the issues of intermixing
semantic information with visible Web elements. A task force had been formed to address theseissues, but there was no W3C standard for including RDF in HTML pages.
One consequence of CC’s limited initial design is that, although millions of Web pages now
include Creative Commons licenses and metadata, there is no uniform, extensible way for tool
developers to access this metadata, and the tools that do exist rely on ad-hoc techniques for
extracting metadata.
Since 2004, Creative Commons has been working with the Web Consortium to create morestraightforward and less limited methods of embedding RDF in HTML documents. These new
methods are now making their way through the W3C standards process. Accordingly,
Creative Commons no longer recommends using RDF/XML in HTML comments for
specifying licensing information. This paper supersedes that recommendation.
We hope that the new ccREL standard presented in this paper will result in a more consistent andstable platform for publishers and tool builders to build upon Creative Commons licenses.

3

The ccREL Abstract Model

This section describes ccREL, Creative Commons’ new recommendation for machine-readable licensing information, in its abstract form, i.e., independent of any concrete syntax. As an abstract
specification, ccREL consists of a small but extensible set of RDF properties that should be provided
with each licensed object. This abstract specification has evolved since the original introductionof CC properties in 2002, but it is worth noting that all first-generation licenses are still correctlyinterpretable against the new specification, thanks in large part to the extensibility properties of
RDF itself.
The abstract model for ccREL distinguishes two classes of properties:
1. Work properties describe aspects of specific works,
including under which license a Work is distributed.
2. License properties describe aspects of licenses.
Publishers will normally be concerned only with Work properties: this is the only information
publishers provide to describe a Work’s licensing terms. License properties are used by Creative
Commons itself to define the authoritative specifications of the licenses we offer. Other organizations
are free to use these components for describing their own licenses. Such licenses, although related
to Creative Commons licenses, would not themselves be Creative Commons licenses nor would they
be endorsed necessarily by Creative Commons.

6

3.1

Work Properties

A publisher who wishes to license a Work under a Creative Commons license must, at a minimum,
provide one RDF triple that specifies the value of the Work’s license property (i.e., the licensethat governs the Work), for example<http://lessig.org/blog/>
xhtml:license<http://creativecommons.org/licenses/by/3.0/> .

Although this is the minimum amount of information, Creative Commons also encourages publishers
to include additional triples giving information about licensed works: the title, the name and URL
for assigning attribution, and the document type. An example might be<http://lessig.org/blog/> dc:title "The Lessig Blog" .
<http://lessig.org/blog/> cc:attributionName "Larry Lessig" .
<http://lessig.org/blog/> cc:attributionURL <http://lessig.org/> .
<http://lessig.org/blog/> dc:type dcmitype:Text .

The specific work properties illustrated here are• dc:title – the document’s title. Here dc: is shorthand for the Dublin Core vocabulary defined at http://purl.org/dc/elements/1.1/ and maintained by the Dublin Core Metadata
Initiative.10• cc:attributionName – the name to cite when giving attribution when the work is modified
or redistributed under the terms of the associated Creative Commons license.11 The prefix
cc:, as mentioned above, is an abbreviation for http://creativecommons.org/ns#.
• cc:attributionURL – the URL to link to when providing attribution.
• dc:type – the type of the licensed document. In this example, the associated value isdcmitype:Text, which indicates text. Lessig’s blog sometimes includes video, in whichcase the type would be dcmitype:MovingImage. Recommended use of dc:type is explained
at http://dublincore.org/documents/dces/. Individual types like dcmitype:Text anddcmitype:MovingImage are part of the DCMI Vocabulary.
Incidentally, the above list of four triples could be alternately expressed using the N3 semicolon
convention, which indicates a list of triples that all have the same subject:
10

The Dublin Core Metadata Initiative (DCMI) promotes the widespread adoption of interoperable metadata
standards, and maintains a vocabulary of DCMI Metadata Terms. See http://dublincore.org/.
11
All current Creative Commons licenses require attribution, and give the publisher the option of specifying a
URL with attribution information. The cc:attributionURL property is the preferred way to provide this URL in
machine-readable form.

7

@prefix dc: <http://purl.org/dc/elements/1.1/>@prefix cc: <http://creativecommons.org/ns#>@prefix dcmitype: <http://purl.org/dc/dcmitype/><http://lessig.org/blog/>
dc:title "The Lessig Blog" ;cc:attributionName "Larry Lessig" ;cc:attributionURL <http://lessig.org/> ;dc:type dcmitype:Text .

There are two more Work properties available to publishers of CC material:• dc:source—indicates the original source of modified work, specified as a URI, for example<http://randomblog.org/modified_lessig_presentation> dc:source <http://lessig.org/> .

• cc:morePermissions—indicates a URL that gives information on additional permissions
beyond those specified in the CC license. For example, a document with a CC license that
requires attribution, might, under certain circumstances, be usable without attribution. Or
a document restricted for noncommercial use could be available for commercial use undercertain conditions.
A typical use would then be:<http://randomblog.org/insightful_posting>
cc:morePermissions <http://randomblog.org/attribution_free_licensing> .

The information at the designated URL is completely up to the publisher, as are the terms of
the associated additional permissions, with one proviso: The additional permissions must be
additional permissions, i.e., they cannot restrict the rights granted by the Creative Commonslicense. Said another way, any use of the work that is valid without taking the morePermissions property into account, must remain valid after taking morePermissions into account.
This is the current set of ccREL Work properties. New properties may be added over time,
defined by Creative Commons or by others. Observe that ccREL inherits the underlying extensibility of RDF—all that is required to create new properties is to include additional triples that
use these. For example, a community of photography publishers could agree to use an additional
photoResolution property, and this would not disrupt the operation of pre-existing tools, so longas the old properties remain available. We’ll see below that the concrete syntax (RDFa) recommended by Creative Commons for ccREL enjoys this same extensibility property.
Distributed creation of new properties notwithstanding, only Creative Commons can include
new elements in the cc: namespace, because Creative Commons controls the defining document
at http://creativecommons.org/ns#. This ability to retain this kind of control, without loss ofextensibility, is a direct consequence of using RDF.
8

3.2

License Properties

We now consider properties used for describing Licenses. With ccREL, Creative Commons does
not expect publishers to use these license properties directly, or even to deal with them at all.
In contrast, Creative Commons’ original metadata recommendation encouraged publishers toprovide the license properties with every licensed work. This design was awkward, because once a
publisher has already indicated which license governs the Work, specifying the license properties
in addition is redundant and thus error prone. The ccREL recommendation does away with thisduplication and leaves it to Creative Commons to provide the license properties.
Tool builders, on the other hand, should take these License properties into account so that they
can interpret the particulars of each Creative Commons license. The License properties governinga Work will typically be found by URL-based discovery. A tool examining a Work notices thexhtml:license property and follows the indicated link to a page for the designated license. Thoselicense description pages—the “Creative Commons Deeds”— are maintained by Creative Commons,and include the license properties in the CC recommended concrete syntax (RDFa), as describedin section 7.2:
Here are the License properties defined as part of ccREL:• cc:permits – permits a particular use of the Work above and beyond what default copyright
law allows.
• cc:prohibits – prohibits a particular use of the Work, specifically affecting the scope of thepermissions provided by cc:permits (but not reducing rights granted under copyright).
• cc:requires – requires certain actions of the user when enjoying the permissions given by
cc:permits.
• cc:jurisdiction – associates the license with a particular legal jurisdiction.
• cc:deprecatedOn – indicates that the license has been deprecated on the given date.
• cc:legalCode – references the corresponding legal text of the license.
Importantly, Creative Commons does not allow third parties to modify these properties for existing
Creative Commons licenses. That said, publishers may certainly use these properties to create newlicenses of their own, which they should host on their own servers, and not represent as being
Creative Commons licenses.
The possible values for cc:permits, i.e., the possible permissions granted by a CC License are:• cc:Reproduction – copying the work in various forms.
• cc:Distribution – redistributing the work.
• cc:DerivativeWorks – preparing derivatives of the work.
The possible values for cc:prohibits, i.e., possible prohibitions that modulate permissions (butdo not affect permissions granted by copyright law, such as fair use) are:• cc:CommercialUse – using the Work for commercial purposes.
9The possible values for cc:requires are:• cc:Notice – providing an indication of the license that governs the work.
• cc:Attribution – giving credit to the appropriate creator.
• cc:ShareAlike – when redistributing derivative works of this work, using the same license.
• cc:SourceCode – when redistributing this work (which is expected to be software when thisrequirement is used), source code must be provided.
For example, the Attribution Share-Alike v3.0 Creative Commons license is described as:12@prefix cc http://creativecommons.org/ns#<http://creativecommons.org/licenses/by-sa/3.0/>
cc:permits cc:Reproduction ;cc:permits cc:Distribution ;cc:permits cc:DerivativeWorks ;cc:requires cc:Attribution ;cc:requires cc:ShareAlike ;cc:requires cc:Notice .

As new copyright licenses are introduced, Creative Commons expects to add new permissions,requirements, and prohibitions. However, it is unlikely that Creative Commons will introduce newlicense property types beyond permits, requires, and prohibits. As a result, tools built tounderstand these three property types will be able to interpret future licenses, at least by listing
the license’s permissions, requirements, and prohibitions: thanks to the underlying RDF frameworkof designating properties by URLs, these tools can easily discover human-readable descriptions ofthese as-yet-undefined property values.

4

Desiderata for concrete ccREL syntaxes

While the previous examples illustrate ccREL using the RDF/XML and N3 notations, ccREL ismeant to be independent of any particular syntax for expressing RDF triples. To create compliant
ccREL implementations, publishers need only arrange that tool builders can extract RDF triples
for the relevant ccREL properties—typically only the Work properties, since Creative Commons
provides the License properties—through a discoverable process. We expect that different publishers will do this in different ways, using syntaxes of their choice that take into account the kinds ofenvironments they would like to provide for their users. In each case, however, it is the publisher’s
responsibility to associate their pages with appropriate extraction mechanisms and to arrange forthese mechanisms to be discoverable by tool builders.
Creative Commons also recommends concrete ccREL syntaxes that tool builders should recognize by default, so that publishers who do not want to be explicitly concerned with extraction
mechanisms have a clear implementation path. These recommended syntaxes—RDFa for HTMLWeb pages, and XMP for free-floating content—are described in the following sections. This section
presents the principles underlying our recommendations.
12

Caveat: The text descriptions of these property values are indicative only. The precise legal interpretations of
the properties can be subtle and even jurisdiction dependent. Consult the full Creative Commons licenses (“legalcode”) for the actual legal definitions.

10

4.1

Principles for HTML

Licensing information for a Web document will be expressed in some form of HTML. What properties would an ideal HTML syntax for expressing Creative Commons terms exhibit? Given theuse cases we’ve observed over the past several years, we can call out the following desiderata:• Independence and Extensibility: We cannot know in advance what new kinds of data wewill want to integrate with Creative Commons licensing data. Currently, we already need to
combine Creative Commons properties with simple media files (sound, images, videos) andthere’s a growing interest in providing markup for complex scientific data (biomedical records,
experimental results). Therefore, the means of expressing the licensing information in HTMLshould be extensible: it should enable the reuse of existing data models and the addition of
new properties, both by Creative Commons and by others. Adding new properties should
not require extensive coordination across communities or approval from a central authority.
Tools should not suddenly become obsolete when new properties are added, or when existing
properties are applied to new kinds of data sets.
• DRY (Don’t Repeat Yourself ): An HTML document often already displays the name of
the author and a clickable link to a Creative Commons license. Providing machine-readable
structure should not require duplicating this data in a separate format. Notably, if the humanclickable link to the license is changed, e.g. from v2.5 to v3.0, a machine processing the pageshould automatically note this change without the publisher having to update another partof the HTML file to keep it “in sync” with the human-readable portion.
• Visual Locality: An HTML page may contain multiple items, for example a dozen photos,
each with its own structured data, for example a different license. It should be easy for tools
to associate the appropriate structured data with their corresponding visual display.
• Remix Friendliness: It should be easy to copy an item from one document and paste it
into a new document with all appropriate structured data included. In a world where weconstantly remix old content to create new content, copy-and-paste, widgets, and sidebars
are crucial elements of the remixable Web. As much as possible, ccREL should allow for easy
copy-and-paste of data to carry along the appropriate licensing information.

4.2

Desiderata for Free-Floating Content

Some important works are not typically conveyed via HTML. Examples are MP3s, MPEGs, andother media files. The technique for embedding licensing data into these files should achieve thefollowing design principles:• Consistency: There are many different possible file types. The mechanism for embedding
licensing information should be reasonably generic, so that a single tool can read and write
the licensing information without requiring awareness of all file types.
• Publisher Accountability: It can be difficult to provide for accountability of licensing
metadata when files are shared in peer-to-peer systems, rather than distributed from a central
location. The method for expressing metadata should facilitate providing publisher accountability at least as strong as the accountability of a Web page with a well-defined host andowner.
11• Simplicity: The process of embedding licensing information should require little more than asimple and free program. In particular, though complex publisher accountability approaches
involving digital signatures and certificates can be used, they should not be required for thebasic use case.

5

Including ccREL information in Web Pages

Consider the abstract model for ccREL. Here, again, are the triples from the Lessig blog example,
expressed in N3.13@prefix xhtml: <http://www.w3.org/1999/xhml#> .
@prefix cc: <http://creativecommons.org/ns#> .
<http://lessig.org/blog/> xhtml:license <http://creativecommons.org/licenses/by/3.0/> .
<http://lessig.org/blog/> cc:attributionName "Lawrence Lessig" .
<http://lessig.org/blog/> cc:attributionURL <http://lessig.org/> .

The Web page to which this information refers typically already contains some HTML that describes
this same information (redundantly), in human-readable form, for example:<div>This page, by<a href="http://lessig.org/">
Lawrence Lessig</a>,is licensed under a<a href="http://creativecommons.org/licenses/by/3.0/">
Creative Commons Attribution License</a>.
</div>

What we would like is a way to quickly augment this HTML with just enough structure toenable the extraction of the RDF triples, using the principles articulated above, including, notably,
Don’t Repeat Yourself: the existing markup and links should be used both for human and machinereadability.

5.1

RDFa and concrete syntax for Work properties

RDFa was designed by the W3C with Creative Commons’ input. The design was motivated inpart by the principles noted above. Using existing HTML properties and a handful of new ones,
RDFa enables a chunk of HTML to express RDF triples, reusing the content wherever possible.
For example, the HTML above would be extended by including additional attributes within theHTML anchor tags as follows:13

It is worth noting that, while the xhtml:license property has long been a part of the CC specification, thecc:attributionName and cc:attributionURL properties are new with ccREL. Under the independence and extensibility principle, the solution we select for embedding ccREL in HTML should allow for such extensions without
breaking tools that already know about xhtml:license.

12

<div about="" xmlns:cc="http://creativecommons.org/ns#">
This page, by<a property="cc:attributionName"
rel="cc:attributionURL" href="http://lessig.org/">
Lawrence Lessig</a>,is licensed under a<a rel="license" href="http://creativecommons.org/licenses/by/3.0/">
Creative Commons Attribution License</a>.
</div>The rules for understanding the meaning of the above markup are as follows:• about defines the subject of all triples within the <div>. Here we have about="", which
defines the subject to be the URL of the current document.
• xmlns:cc associates, throughout the <div>, the prefix cc with the URL http://creativecommons.
org/ns#, much as N3 does with @prefix.
• property generates a new triple with predicate cc:attributionName, and the text content
of the element, in this case “Lawrence Lessig,” as the object.
• rel="cc:attributionURL" generates a new triple with predicate cc:attributionURL, andthe URL in the href as the object.
• rel="license" generates a new triple with predicate xhtml:license, as xhtml is the default
prefix for reserved XHTML values like license. The object is given by the href.
The fragment of HTML (within the div) is entirely self-contained (and thus remix-friendly).
Its meaning would be preserved if it were copied and pasted into another Web page. The data’s
structure is local to the data itself: a human looking at the page could easily identify the structured
data by pointing to the rendered page and finding the enclosing chunk of HTML. In addition,
the clickable links and rendered author names gain semantic meaning without repeating the coredata. Finally, as this is embedded RDF, the extensibility and independence properties of RDF
vocabularies are automatically inherited: anyone can create a new vocabulary or reuse portions ofexisting vocabularies.
Of course, one can continue to add additional data, both visible and structured. Figure 2
shows a more complex example that includes all Work properties currently supported by Creative
Commons, including how this HTML+RDFa would be rendered on a Web page. Notice howproperties can be associated with HTML spans as well as anchors, or in fact with any HTMLelements—see the RDFa specification for details.
The examples in this section illustrate how publishers can specify Work properties. One canalso use RDFa to express License properties. This is what Creative Commons does with the license
description pages on its own site, as described below in section 7.2.

13

<div about="" instanceof="cc:Work"
xmlns:cc="http://creativecommons.org/ns#"
xmlns:dc="http://purl.org/dc/elements/1.1/"
align="center"><img alt="Creative Commons License"
src="http://i.creativecommons.org/l/by/3.0/us/88x31.png" /></a><br /><span property="dc:title">The Lessig Blog</span>,a<span rel="dc:type" href="http://purl.org/dc/dcmitype/Text">
collection of texts</span>by<a property="cc:attributionName"
rel="cc:attributionURL" href="http://lessig.org/">
Lawrence Lessig</a>,<br />is licensed under a<a rel="license" href="http://creativecommons.org/licenses/by/3.0/">
Creative Commons Attribution License</a>.<br />There are<a rel="cc:morePermissions"
href="http://lessig.org/blog/other-license">
alternative licensing options</a>.
</div>

Figure 2: RDFa markup of a Creative Commons license notice, illustrating all the current CC Work
properties, including the rendering of this markup in a Web browser.

14

5.2

Microformats

Microformats are a set of simple, open data formats “designed for humans first and machines
second.” They provide domain-specific syntaxes for annotating data in HTML. At the moment,
the two widely deployed “compound” microformats annotate contact information (hCard) andcalendar events (hCal). Of the “elemental” microformats, those meant to annotate a single data
point, the most popular is rel-tag, used to denote a “tag” on an item, e.g. a blog post. Anotherelemental microformat is rel-license, which is meant to indicate the current page’s license andwhich, conveniently, uses a syntax which overlaps with RDFa: rel="license". Other microformats
may, over time, integrate Creative Commons properties, for example when licensing images, videos,
and other multimedia content.14
Microformat designers have focused on simplicity and readability, and Creative Commons encourages publishers who use microformats to make it easy for tool builders to extract the relevant
ccREL triples. Nonetheless, microformats’ syntactic simplicity comes at the cost of independence
and extensibility, which makes them limited from the Creative Commons perspective.
For example, every time a Creative Commons license needs to be expressed in a new context—
e.g. videos instead of still images—a new microformat and syntax must be designed, and all parsers
must then, somehow, become aware of the change. It is also not obvious how one might combine
different microformats on a single Web page, given that the syntax rules may differ and evenconflict from one microformat to the next.15 Finally, when it comes time to express complex data
sets with ever expanding sets of properties, e.g., scientific data, microformats do not appear to scale
appropriately, given their lack of vocabulary scoping and general inability to mix vocabularies fromindependently developed sources—the kind of mixing that is enabled by RDF’s use of namespaces.
Thus, Creative Commons does not recommend any particular microformat syntax for ccREL,
but we do recommend a method for ensuring that, when publishers use microformats, tool builders
can extract the corresponding ccREL properties: use an appropriate profile URL in the header ofthe HTML document.16 This profile URL significantly improves the independence and extensibilityof microformats by ensuring that the tools can find the appropriate parser code for extracting the
ccREL abstract model from the microformat, without having to know about all microformats inadvance. One downside is that the microformat syntax then becomes less remix-friendly, with two
disparate fragments: one in the head to declare the profile, and one in the body to express thedata. Even so, the profile approach is likely good enough for simple data. It is worth noting that
this use of a profile URL is already recommended as part of microformats’ best practices, though
it is unfortunately rarely implemented today in deployed applications.

5.3

GRDDL for XML Documents

Not all documents on the web are HTML: one popular syntax for representing structured data in
XML. Given that XML is a machine-readable syntax, often with a strict schema depending on thetype of data expressed, not all of the principles we outlined are useful here. In particular, visual14

See http://microformats.org/.
See http://microformats.org/wiki/grouping-brainstorming for one discussion.
16
Profile URLs indicate that the HTML file can be interpreted according to the rules of that profile. This property
has been used by some microformat specifications to indicate, e.g., “this page contains the hCard microformat”. The
property is also used by GRDDL for generic HTML transformations to RDF/XML, though this approach to RDF
extraction from HTML is not fully compliant with the principles laid out in this paper: it is difficult to tell which
image on a page is CC-licensed when the RDF extraction is achieved via GRDDL.
1515

locality is not relevant when the reader is a machine rather than a human, and remix-friendlinessdoesn’t really apply when XML fragments are rarely remixable in the first place, given schema
validation. Thus, we focus on independence and extensibility, as well as DRY.
When publishing Creative Commons licensing information inside an XML document, Creative
Commons recommends exposing a mechanism to extract the ccREL abstract model from the XML,so that Creative Commons tools need not know about every possible XML schema ahead of time.
The W3C’s GRDDL recommendation performs exactly this task by letting publishers specify, either in each XML document or once in an XML schema, an XSL Transformation that extracts
RDF/XML from XML.17 Consider, for example, a small extension of the Atom XML publishing
schema for news feeds:18<entry><title>Lessig 2.0 -- the site</title><link rel="alternate" type="text/html"
href="http://lessig.org/blog/2007/06/lessig_20_the_site.html" /><id>tag:lessig.org,2007:/blog//1.3401</id><published>2007-06-25T19:44:48Z</published><link rel="license" type="text/html"
href="http://creativecommons.org/licenses/by/3.0/us/" /></entry>

An appropriate XSL Transform can easily process this data to extract the ccREL property thatspecifies the license:<rdf:RDF about="http://lessig.org/blog/2007/06/lessig_20_the_site.html"
xmlns:cc="http://creativecommons.org/ns#"><cc:license resource="http://creativecommons.org/licenses/by/3.0/us/" /></rdf:RDF>

Similarly, the Open Archives Initiative, defines a complex XML schema for library resources.19
These resources may include megabytes of data, including sometimes the entire resource in full text.
Using XSLT, one can extract the relevant ccREL information, exactly as above. Using GRDDL, theOpen Archives Initiative can specify the XSLT in its XML schema file, so that all OAI documents
are automatically transformable to RDF/XML, which immediately conveys ccREL.
Direct RDF/XML embedding in XML. Interestingly, because RDF can be expressed using
the RDF/XML syntax, one might be tempted to use RDF/XML directly inside an XML documentwith an appropriate schema definition that enables such direct embedding. This very approachis taken by SVG,20 and there are cases of SVG graphics that include licensing information using
directly embedded RDF/XML.
This approach can be made ccREL compliant with very little work—a simple GRDDL transform, declared in the XML schema definition, that extracts the RDF/XML and expresses it on its
17

Gleaning Resource Descriptions from Dialects of Languages (GRDDL) (http://www.w3.org/TR/grddl/) is a
W3C recommendation for linking Web documents to algorithms that extract RDF data from the document.
18
Atom License Extension. See http://tools.ietf.org/html/rfc4946.
19
http://www.openarchives.org/
20
Scalable Vector Graphics, http://www.w3.org/Graphics/SVG/, a W3C Recommendation for vector graphics
expressed using XML.

16

own. Note that, for ccREL compliance, this transform, although simple, is necessary. The reason
for its necessity goes to the crux of the ccREL principles: without such a transform provided byeach XML schema designer, tools would have to be aware of all the various XML schemas that
include RDF/XML in this way. For extensibility and future-proofing, ccREL asks that publishersof the schema make the effort to provide the extraction mechanism. With explicit extraction mechanisms, publishers have a little bit more work to do, while tool builders are immediately empowered
to create generic programs that can process data they have never seen before.

6

Embedding ccREL in Free-Floating Files

We turn to the precise Creative Commons recommendation for embedding ccREL metadata inside
MP3s, Word documents, and other “free-floating” content that is often passed around in a peerto-peer fashion, via email or P2P networks. We note that there are two distinct issues to resolve:• Expression: expressing the abstract model using a specific syntax and embedding, and• Accountability: providing minimal accountability for the expressed ccREL data.
We handle accountability for free-floating content by connecting any free-floating document
to a Web page, and placing the ccREL information on that Web page. Thus, publishers of freefloating content are just as accountable as publishers of Web-based content: rights are always
expressed on a Web page. The connection between the Web page and the binary file it describes
is achieved using a cryptographic hash, i.e. a fingerprint, of the file. For example, the PDF fileof Lawrence Lessig’s “Code v2” will contain a reference to http://codev2.cc/download+remix,
which itself will contain a reference to the SHA1 hash of the PDF file. The owner of the URL http://codev2.cc/download+remix is thus taking responsibility for the ccREL statements it makes
about the file.
For expression, we recommend XMP. XMP has the broadest support of any embedded metadata
format (perhaps it is the only such format with anything approaching broad support) across manydifferent media formats. With the exception of media formats where a workable embedded metadata
format is already ubiquitous (e.g. MP3), Creative Commons recommends adopting XMP as anembedded metadata standard and using the following two fields in particular:• Web reference: value of xapRights:WebStatement• License: value of cc:license
Consider our example of Lessig’s “Code v2”, a Creative Commons licensed, community-edited
second version of his original “Code and Other Laws of Cyberspace.” The PDF of this book,available at http://pdf.codev2.cc/Lessig-Codev2.pdf, contains XMP metadata as follows:17

<?xpacket begin="" id=""?><x:xmpmeta xmlns:x="adobe:ns:meta/"><rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"><rdf:Description rdf:about=""
xmlns:xapRights="http://ns.adobe.com/xap/1.0/rights/"><xapRights:Marked>True</xapRights:Marked><xapRights:WebStatement rdf:resource="http://codev2.cc/download+remix/" /></rdf:Description>...
<rdf:Description rdf:about=""
xmlns:cc="http://creativecommons.org/ns#"><cc:license rdf:resource="http://creativecommons.org/licenses/by-sa/2.5/" /></rdf:Description></rdf:RDF></x:xmpmeta>

Notice how this is RDF/XML, including a xapRights:WebStatement pointer to the web page
http://codev2.cc/download+remix/, which itself contains RDFa:
Any derivative must be licensed under a<a about="urn:sha1:W4XGZGCD4D6TVXJSCIG3BJFLJNWFATTE"
rel="license"
href="http://creativecommons.org/licenses/by-sa/2.5/">
Creative Commons Attribution-ShareAlike 2.5 License</a>.

This RDFa references the PDF using its SHA1 hash—a secure fingerprint of the file that matches
only the given PDF file—and declares its Creative Commons license. Thus, anyone that finds the“Code v2” PDF can find its WebStatement pointer, look up that URL, verify that it properlyreferences the file via its SHA1 hash, and confirm the file’s Creative Commons license on the
web-based deed.

7

Examples and Use Cases

This section describes several examples, first by publishers of Creative Commons licensed works,
then by tool builders who wish to consume the licensing information. Some of these examples
include existing, real implementations of ccREL, while others are potential implementations andapplications we believe would significantly benefit from ccREL.

7.1

How Publishers Can Use ccREL

Publishers can mix ccREL with other markup with great flexibility. Thanks to ccREL’s independence and extensibility principle, publishers can use ccREL descriptions in combination withadditional attributes taken from other publishers, or with entirely new attributes they define for
their own purposes. Thanks to ccREL’s DRY principle, even small publishers get the benefit of
updating data in one location and automatically keeping the human- and machine-readable in sync.

18

<div class="mediaDetails haudio"
xmlns:xsd="http://www.w3.org/2001/XMLSchema"
xmlns:dc="http://purl.org/dc/elements/1.1/"
xmlns:commerce="http://purl.org/commerce/elements/1.0/"
xmlns:hmedia="http://purl.org/hmedia/elements/1.0/"
about="#album-6579151">...
<a id="mediaImageLink" rel="hmedia:depiction"
href="http://www.bitmunk.com/view/image/6579151">...
<h1 property="dc:title" class="mediaTitle album fn">Lifeseeker</h1>...
<span property="dc:creator" class="fn">Lifeseeker</span>...
<span property="dc:contributor" class="fn">(P) 2005 One In A Million Records</span>...
<span property="dc:date" class="published" title="2007-11-18T11:23:07-05:00"
content="2007-11-18T11:23:07-05:00" datatype="xsd:date">
2002-07-23</span>...
<a href="/browse/genre/audio_album/59"
property="dc:type" class="category">Hip Hop and Rap</a><span class="detailLabel">Tracks:</span>
16 (<abbr property="hmedia:duration" class="duration"
title="PT1H13M37S" content="PT1H13M37S"
datatype="xsd:duration">1:13:37</span>)...
<span class="detailLabel">Licenses:</span><img property="dc:license" class="licenseIcon"
src="/themes/bm2/images/licenses/sc-sm.png"
alt="Standard Copyright" title="Standard Copyright"
content="Standard Copyright"/>...
</div>

Figure 3: Markup for a Bitmunk Song: this is a real excerpt of the actual HTML markup used on
the bitmunk.com web site, slightly simplified and indented for readability.

19

Mixing Content with Different Licenses
A common use case for Web publishers working in a mashup-friendly world is the issue of mixing
content with different licenses. Consider, for example, what happens if Larry Lessig’s blog reuses
an image published by another author and licensed for non-commercial use. Recall that Lessig’s
Blog is licensed to permit commercial use.
The HTML markup in this case is straightforward:<div about="" xmlns:cc="http://creativecommons.org/ns#">
This page, by<a property="cc:attributionName"
rel="cc:attributionURL" href="http://lessig.org/">
Lawrence Lessig</a>,is licensed under a<a rel="license" href="http://creativecommons.org/licenses/by/3.0/">
Creative Commons Attribution License</a>.
<div about="/photos/constitution.jpg">
The photo of the constitution used in this post was originally published by<a rel="dc:source" href="http://example.org/">Joe Example</a>, and is licensed under a<a rel="license" href="http://creativecommons.org/licenses/by-nc/3.0/">
Creative Commons Attribution-NonCommercial License</a>.
</div></div>The inner <div> uses the about attribute to indicate that its statements concern the photo inquestion. A link to the original source is provided using the dc:source property, and a different
license pointer is given for this photo using the normal anchor with a rel="license" attribute.
hAudioBitmunk is a service that supports artists with a legal, copyright-aware, content distribution service.
The service needed a mechanism for embedding structured data about songs and albums directly
into their web pages, including licensing information, so that browser add-ons might provide additional functionality around the music, e.g. comparing the price of a particular song at various online
stores. Bitmunk first created a microformat called hAudio. They soon realized, however, that theywould be duplicating fields when it came time to define hVideo, and that these duplicated fields
would no longer be compatible with those of hAudio. More immediately problematic, hAudio’s
basic fields, e.g. title, would not be compatible with other “title” fields of other microformats.
Thus, Bitmunk created the hAudio RDFa vocabulary. The design process for this vocabulary
immediately revealed separate, logical components: Dublin Core for basic properties e.g. title,
Creative Commons for licensing, a new vocabulary called “hMedia” for media-specific properties
e.g. duration, and a new vocabulary called “hCommerce” for transaction-specific properties e.g.
price. Bitmunk was thus able to reuse two existing vocabularies and add features. It was also able
to clearly delineate logical components to make it particularly easy for other vocabulary developers
to reuse only certain components of the hAudio vocabulary, e.g. hCommerce. Meanwhile, all
Creative Commons licensing information is still expressible without alteration.
20

Figure 3 shows an excerpt of the markup available from Bitmunk at http://bitmunk.com/
view/media/6579151. Note that this particular sample is not CC-licensed: it uses standard copyright. A CC-licensed album would be marked up in the same way, with a different license value:
Bitmunk was able to develop its vocabulary independent of ccREL, and can now integrate with
ccREL simply by adding the appropriate attributes.
FlickrFlickr hosts approximately 50 million CC-licensed images (as of October 2007). Currently Flickr
denotes a license on each image’s page with a link to the relevant license qualified by rel="license".
This ad-hoc convention, encouraged by the microformats effort, was “grandfathered” into RDFa
thanks to the reserved HTML keyword license. Unfortunately, it works only for simple use cases,
with a single image on a single page. This approach breaks down when multiple images are viewed
on a single page, or when further information, such as the photographer’s name, is required.
Flickr could significantly benefit from the ccREL recommendations, by providing, in addition
to simple license linking:• License assertions scoped to the image being licensed.
• Attribution details.
• A cc:additionalPermissions reference to commercial licensing brokers and a dc:source
reference to parent works.
• XMP embedding in images themselves.
In addition, Flickr recently deployed “machine tags,” where photographers can add metadata
about their images using custom properties. Flickr’s machine tags are, in fact, a subset of RDF,
which can be represented easily using RDFa. Thus, Creative Commons licensing can be easily
expressed alongside Flickr’s machine tags using the same technology, without interfering.
Figure 4 shows how the CC-licensed photo at http://www.flickr.com/photos/laughingsquid/
2034629532/ would be marked up using ccREL, including the machine tag upcoming:event thatassociates the photo with an event at http://upcoming.org.
Nature Precedings
Nature, one of the world’s top scientific journals, recently launched a web-only “precedings” site,where early results can be announced rapidly in advance of full-blown peer review. Papers on
Nature Precedings are distributed under a Creative Commons license. Like Flickr, Nature Precedings
currently uses CC’s prior metadata recommendation: RDF/XML included in an HTML comment.
Nature could significantly benefit from the ccREL recommendation, which would let them publish
structured Creative Commons licensing information in a more robust, more extensible, and morehuman-readable way.
Consider, for example, the Nature Preceding paper at http://precedings.nature.com/documents/
1290/version/1. Figure 5 shows how the markup at that page can be extended with simple RDFa

21

<div xmlns:dc="http://purl.org/dc/elements/1.1/"
xmlns:cc="http://creativecommons.org/ns#"
xmlns:flickr="http://flickr.com/ns#"
about="http://www.flickr.com/photos/laughingsquid/2034629532/">...
<h1 property="dc:title">NewTeeVee Live Game Show</h1>...
<img rel="flickr:defaultPhoto"src="http://farm3.static.flickr.com/2320/2034629532_02085434dd.jpg?v=0" />...
<div property="dc:description">
See the blog post for more info:<a href="http://laughingsquid.com/a-few-random-newteevee-live-photos/">
A Few Random NewTeeVee Live Photos</a></div>...
This photo is licensed under a<a rel="license" href="http://creativecommons.org/licenses/by-nc-nd/2.0/">
Creative Commons license</a>.
If you use this photo within the terms of the license or makespecial arrangements to use the photo, please list the photo credit as<span property="cc:attributionName">Scott Beale / Laughing Squid</span>and link the credit to<a rel="cc:attributionURL" href="http://laughingsquid.com">laughingsquid.com
</a>.
...
Uploaded on<span property="flickr:uploaded" content="2007-11-15">
November 15, 2007</span>...
<h4>Tags</h4><a rel="flickr:tag" href="/photos/laughingsquid/tags/newteevee/">NewTeeVee</a><a rel="flickr:tag" href="/photos/laughingsquid/tags/gigaom/">GigaOm</a>...
<a rel="upcoming:event" href="http://upcoming.org/event/286436">upcoming:event=286436</a>...
</div>

Figure 4: A Flickr Photo Page with RDFa: this is an excerpt from a Flickr photo page with small
amounts of additional markup to show how one would integrate RDFa. The rendering of the HTMLis identical with the added RDFa properties. Note the Flickr machine tag upcoming:event, whichreferences an event at upcoming.org. This machine tag is, in fact, an RDF triple, easily expressedin RDFa alongside existing Flickr information and CC licensing.
22

<div xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:cc="http://creativecommons.org/ns#"
xmlns:prism="http://prismstandard.org/namespaces/1.2/basic/"
xmlns:foaf="http://xmlns.com/foaf/0.1/" about="/documents/1290/version/1"><h2 property="dc:title">An Olfactory Receptor Pseudogene whose Function emerged in Humans</h2>...
<span rel="dc:creator"><span property="foaf:name">Peter Lai</span></span> <sup>1</sup>,<a rel="dc:creator" href="http://precedings.nature.com/account/show/1068"><span property="foaf:name">Gautam Bahl</span></a><sup>2</sup>,...
<dt class="doctype">Document Type:</dt><dd property="dc:type">Manuscript</dd>
Received <span property="dc:date">02 November 2007 21:20 UTC</span>;
Posted <span property="prism:publicationDate">05 November 2007</span>...
<a rel="prism:category" href="http://precedings.nature.com/subjects/biotechnology">
Biotechnology
</a>,
...
<ul id="revision-1321-tags" class="taglist"><li> <a rel="nature:tag" href="http://precedings.nature.com/tags/olfactory+receptors">
olfactory receptors
</a>
</li>
...
</ul>
...
This document is licensed to the public under the<a rel="license" href="http://creativecommons.org/licenses/by/2.5/">
Creative Commons Attribution 2.5 License</a>...
<!-- Citation --><dt class="abstract">How to cite this document:</dt><dd><p> <span property="cc:attributionName">Lai, Peter, Bahl, Gautam, Gremigni, Maryse, Matarazzo, Valery ,Clot-Faybesse, Olivier, Ronin, Catherine, and Crasto, Chiquito.
An Olfactory Receptor Pseudogene whose Function emerged in Humans.
</span>Available from Nature Precedings &#060;<a rel="cc:attributionURL" href="http://dx.doi.org/10.1038/npre.2007.1290.1">
http://dx.doi.org/10.1038/npre.2007.1290.1</a>&#062; (2007)
</p>
</dd>
...
</div>

Figure 5: Markup for a Nature Precedings article, including how RDFa might be integrated seamlessly into the existing markup. The property nature:tag is used to indicate a Nature-defined wayof tagging content, though another vocabulary could easily be used here.
23

Figure 6: Portions of a Nature Precedings paper, marked up with RDFa. An RDFa-aware browser(in this case any normal browser using the RDFa Bookmarklets) detects the markup, highlighting
the title and Creative Commons license, and revealing the corresponding RDF triples.
attributes, using the Dublin Core, Creative Commons, FOAF, and PRISM publication vocabularies.21 Notice how any HTML element, including the existing H1 used for the title, can be used tocarry RDFa attributes. Figure 6 shows how this page could appear in an RDFa-aware browser.
Scientific Data
Open publication of scientific data on the Internet has begun, with the Nature Publishing Group
recently announcing the release of genomic data sets under a Creative Commons license.22 Beyond
simple licensing, thousands of new metadata vocabularies and properties are being developed toexpress research results. Creative Commons, through its Science Commons subsidiary,23 is playing
an active role here, working to remove barriers to scientific cooperation and sharing. Science
21

The Publishing Requirements for Industry Standard Metadata (PRISM) provides a vocabulary for publishing
and aggregating content from books, magazines, and journals. See http://www.prismstandard.org/.
22
See http://www.nature.com/authors/editorial_policies/license.html for details.
23
See http://sciencecommons.org

24Commons is specifically encouraging the creation of RDF-based vocabularies for describing scientificinformation and is stimulating collaboration among research communities with tools that build on
RDF’s extensibility and interoperability.

Figure 7: A simple rendering of a bibliographic entry with extra scientific data.
As these vocabularies become more widespread, it’s easy to envision uses of ccREL and RDFa
that extend the bibliographic and licensing markup to include these new scientific data tags. Tools
may then emerge to take advantage of this additional markup, enabling dynamic, distributed
scientific collaboration through interoperable referencing of scientific concepts.
Imagine, for example, an excerpt from a (hypothetical) Web-based newsletter about genomics
research, which references an (actual) article from BioMed Central Neurosciences, as it might berendered by a browser (Figure 7). The words “recent study on rat brains”, and “CEBP-beta” areclickable links, leading respectively, to a Web page for the paper, and a Web page that describes
the protein CEBP-β in the Uniprot protein database.
The RDFa generating this excerpt could be<div
xmlns:OBO_REL="http://www.obofoundry.org/ro/ro.owl#"
xmlns:UNIPROT="http://purl.uniprot.org/uniprot/">
A <a href="http://www.biomedcentral.com/1471-2202/6/69">
recent study on rat brains</a>by von Gertten et. al. reports that<div about="http://purl.org/obo/owl/GO#GO_0050729"><span property="rdfs:label">inflammatory stimuli</span>
upregulate expression of<a rel="OBO_REL:precedes"
href="http://purl.uniprot.org/uniprot/P17676"><span property="rdfs:label">CEPB-beta</span></a></div></div>

25

This RDFa not only links to the paper in the usual way, but it also provides machine-readable
information that this is a statement about inflammatory stimuli (as defined by the Open Biomedical Ontologies initiative) activating expression of the CEPB protein (as specified in the UniProt
database of proteins). Since the URI of the protein is visually meaningful, it can be marked up
with a clickable link that also provides the object of a triple.
Additional Permissions
A CC license grants certain permissions to the public; others may be available privately. A coarsegrained “more permissions” link indicates this availability. Creative Commons has branded thisscheme CC+. Also, since CC licenses are non-exclusive, other options for a work may be offeredin addition to a CC license. Here is an example from http://magnatune.com, showing the use of
RDFa to annotate the standard CC license image and also the Magnatune logo:<a href="http://creativecommons.org/licenses/by-nc-sa/1.0/" rel="license"><img src="http://he3.magnatune.com/img/somerights2.gif"></a><a href="https://magnatune.com/artists/license/?artist=Anup&album=Embrace&genre=World"
xmlns:cc="http://creativecommons.org/ns#" rel="cc:morePermissions"><img border=0 src="http://he3.magnatune.com/img/button_license2.gif"></a>This snippet contains two statements: the public CC license and the availability of more permissions. Sophisticated users of this protocol will one day publish company, media, or genre-specificdescriptions of the permissions available privately at the target URL. Tools built to recognize a
Creative Commons license will still be able to detect the Creative Commons license after the addition of the morePermissions property, which is exactly the desired behavior. More sophisticatedversions of the tools could inform the user that “more permissions” may be granted by following
the indicated link.

7.2

Publishing license properties

As mentioned above, Creative Commons doesn’t expect content publishers to deal with licenseproperties. However, others may find themselves publishing licenses using ccREL’s license properties. Here, too, RDFa is available as a framework for creating license descriptions that arehuman-readable, from which automated tools can also extract the required properties.
One example of this is Creative Commons itself, and the publication of the “Commons Deeds”.
Figure 8 shows the HTML source of the Web page at http://creativecommons.org/licenses/
by-nd/3.0/us/ which describes the U.S. version of the CC Attribution-NoDerivatives license. As
this markup shows, any HTML attribute, including LI, can carry RDFa attributes. The href
attribute, typically used for clickable links, can be used to indicate a structured relation, evenwhen the element to which it is attached is not an HTML anchor.
In this markup, the “Attribution-NoDerivatives” license permits distribution and reproduction,
while requiring attribution and notice. Recall that ccREL is meant to be interpreted in addition to
the baseline copyright regulation. In other words, the restriction “NoDerivatives” is not expressed26<h3>You are free:</h3><ul><li class="license share"><strong>to Share</strong> -to<span rel="cc:permits"
href="http://creativecommons.org/ns#Distribution">copy</span>,<span rel="cc:permits"
href="http://creativecommons.org/ns#Reproduction">distribute</span>,display, andperform the work</li>
<div id="deed-conditions"><h3>Under the following conditions:</h3><ul align="left" dir=""><li rel="cc:requires"
href="http://creativecommons.org/ns#Attribution" class="license by"><p><strong>Attribution</strong>.
<span id="attribution-container">
You must attribute the work in the manner specified by
the author or licensor(but not in any way thatsuggests that they endorse you or your use of the work).
</span></li><li class="license nd"><p><strong>No Derivative Works</strong>.
<span>You may not alter, transform, or build upon this work.
</span></li><li rel="cc:requires"
href="http://creativecommons.org/ns#Notice">
For any reuse or distribution, you must make clear toothers the license terms of this work. The best way to
do this is with a link to this web page.</li></ul></div></ul>

Figure 8: Part of the HTML code for the Creative Commons Attribution, No Derivatives Deed(slightly simplified for presentation purposes) showing the use of ccREL License Properties.

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in ccREL, since that is already a default in copyright law. The opposite, where derivative works
are allowed, would be denoted with an additional CC permission.
Tool builders who then want to extract RDF from this page can do so using, for example, theW3C’s RDFa Distiller,24 which, when given the CC Deed URL http://creativecommons.org/
licenses/by-nd/3.0/, produces the RDF/XML serialization of the same structured data, readyto be imported into any programming language with RDF/XML support:<?xml version="1.0" encoding="utf-8"?><rdf:RDF
xmlns:cc="http://creativecommons.org/ns#"
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"><rdf:Description rdf:about="http://creativecommons.org/licenses/by-nd/3.0/"><cc:requires rdf:resource="http://creativecommons.org/ns#Notice"/><cc:requires rdf:resource="http://creativecommons.org/ns#Attribution"/><cc:permits rdf:resource="http://creativecommons.org/ns#Distribution"/><cc:permits rdf:resource="http://creativecommons.org/ns#Reproduction"/></rdf:Description></rdf:RDF>

7.3

How Tool Builders Can Use ccREL

MozCC
MozCC25 is an extension to Mozilla-based browsers for extracting and displaying metadata embedded in web pages. MozCC was initially developed in 2004 as a work-around to some of the
deficiencies in the prior Creative Commons metadata recommendation. That version of MozCC
specifically looked for Creative Commons RDF in HTML comments, a place most other parsers
ignore. Once the metadata detected, MozCC provided users with a visual notification, via icons inthe status bar, of the Creative Commons license. In addition, MozCC provided a simple interface
to expose the work and license properties.
Since the initial development, MozCC has been rewritten to provide general purpose extractionof all RDFa metadata, as well as a specialized interface for ccREL. The status-bar icons anddetailed metadata visualization features have been preserved and expanded. A MozCC user receives
immediate visual cues when he encounters a page with RDFa metadata, including specific CCbranded icons when the metadata indicates the presence of a Creative Commons license. The
experience is pictured in Figure 9.
MozCC processes pages by listening for load events and then calling one or more metadata
extractors on the content. Metadata extractors are JavaScript classes registered on browser startup;
they may be provided by MozCC or other extensions. MozCC ships with extractors for all current
and previous Creative Commons metadata recommendations, in particular ccREL. Each registeredextractor is called for every page. The extractors are passed information about the page to beprocessed, including the URL and whether the page has changed since it was last processed. This
allows individual extractors to determine whether re-processing is needed. The RDFa extractor,
for example, can stop processing if it sees the document hasn’t been updated. An extractor which
2425

http://www.w3.org/2007/08/pyRdfa/
http://wiki.creativecommons.org/MozCC

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Figure 9: The MozCC Mozilla Add-On. The status bar shows a CC icon that indicates to theuser that the page is CC-licensed. A click on the icon reveals the detailed metadata in a separate
window.

29

looks for metadata specified in external files via <link> tags, however, would still retrieve themand see if they have been updated.
The results of each extractor are stored in a local metadata store. In the case of Firefox, thisis a SQLite database stored as part of the user’s profile. The local metadata store serves as an
abstraction layer between the extractors and user interface code. The contents are visible throughthe Page Info interface. The current software only exposes this information as status bar icons;
one can imagine other user interfaces (provided by MozCC or other extensions) which expose themetadata in different ways.
Operator
Operator26 is an add-on to the Firefox browser that detects microformats and RDFa in the web
pages a user visits. Operator can be extended with “action scripts” that are triggered by specificdata found in the web page. The regions of the page that contain data are themselves highlighted
so that users can visually detect and receive contextual information about the data.
It is relatively straight-forward to write a Creative Commons action script that finds all Creative
Commons licensed content inside a web page by looking for the RDFa syntax. This allows users
to easily identify their rights and responsibilities when reusing content they find on the web. No
matter the item, even types of items with properties currently unanticipated, the simple action
script can detect them, display the item’s name and rights description.
Putting aside for now the definition of some utility functions, an action handler for the licenseproperty is declared as follows:27
RDFa.DEFAULT_NS.cc = "http://creativecommons.org/ns#";
RDFa.ns.cc = function(name) { return RDFa.DEFAULT_NS.cc + name; };var view_license = {description: "View License",
shortDescription: "View",
scope: {semantic: {"RDF" : {property : RDFa.ns.cc("license"),defaultNS : RDFa.ns.cc("")}}},
doAction: function(semanticObject, semanticObjectType, propertyIndex) {if (semanticObjectType == "RDF") {return semanticObject.license;}}};
SemanticActions.add("view_license", view_license);

26

https://addons.mozilla.org/en-US/firefox/addon/4106
Operator currently does not handle HTML reserved keywords, such as rel="license". Thus, we consider the
script for the property cc:license, and provide examples appropriately adjusted. This gap is expected to be filledby early 2008.
27

30

Figure 10: Operator with a CC action script on Lessig’s Blog. Notice the two resources, each withits “view license” action.
Once this action script enabled, Operator automatically lights up Creative Commons licensed“Resources” it finds on the web. For example, browsing to the Lessig blog, Operator highlights
two resources that are CC-licensed: the Lessig Blog itself, and a Creative Commons licensed photoused in one of the blog posts. The result is shown in Figure 10.

8Conclusion

Creative Commons wants to make it easy for artists and scientists to build upon the works of others
when they choose to: licensing your work for reuse and finding properly licensed works to reuseshould be easy. To achieve this on the technical front, we have defined ccREL, an abstract model
for rights expression based on the W3C’s RDF, and we recommend two syntaxes for web-based andfree-floating content: RDFa and XMP, respectively. The major goal of our technological approach31

is to make it easy to publish and read rights expression data now and in the future, when the kindsof licensed items and the data expressed about them goes far beyond what we can imagine today.
By using RDF, ccREL links Creative Commons to the fast-growing RDF data interoperability
infrastructure and its extensive developer toolset: other data sets can be integrated with ccREL,and RDF technologies, e.g. data provenance with digital signatures, can eventually benefit ccREL.
We believe that the technologies we have selected for ccREL will enable the kind of powerful,
distributed technological innovation that is characteristic of the Internet. Anyone can create new
vocabularies for their own purposes and combine them with ccREL as they please, without seeking
central approval. Just as we did with the legal text of the licenses, we aim to create the minimal
infrastructure required to enable collaboration and invention, while letting it flourish as an organic,
distributed process. We believe ccREL provides this primordial technical layer that can enable avibrant application ecosystem, and we look forward to the community’s innovative ideas that can
now freely build upon ccREL.

9

Acknowledgments

The authors wish to credit Neeru Paharia, past Executive Director of Creative Commons, for the“free-floating” content accountability architecture, Manu Sporny, CEO of Bitmunk, for the Creative
Commons Operator code, and Aaron Swartz for the original Creative Commons RDF data modeland metadata strategy. More broadly, the authors wish to acknowledge the work of a number of
W3C groups, in particular all members of the RDF-in-HTML task force (Mark Birbeck, Jeremy
Carroll, Michael Hausenblas, Shane McCarron, Steven Pemberton, and Elias Torres), the Semantic
Web Deployment Working Group chaired by Guus Schreiber and David Wood, and the tirelessW3C staff without whom there would be no RDFa, GRDDL, or RDF, and thus no ccREL: Eric
Miller, Ralph Swick, Ivan Herman, and Dan Connolly.

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