In the age of online catalogs the content of catalog records must use some kind of electronic encoding scheme in order to be machine-manipulable for searching and display.
Encoding allows each part of a record to be set off from every other part.
Then computer programs can be written so that each part will be displayed in a certain position according to the wishes of the programmer. An excellent history of its development may be found in Deborah Byrne s MARC Manual . Other encoding systems have been developed recently in response to the desire to make catalog records available on the Web. It is a set of rules for designing markup languages that describe the structure of a document. A DTD for MARC records has been developed, and at this writing is in experimental stages.