Copyright is a type of legal protection based on the United States Constitution for original works of authorship fixed in a tangible medium of expression. Copyright protection extends to both published and unpublished works. An unpublished work is one that has not been published or otherwise made available to the public. Unpublished works may be protected by copyright, but only if they meet certain requirements. For example, they must be created within the applicable copyright period, and they must not be used without the express permission of the owner. Many artists' estates enforce copyrights on their creations. These copyrights often include provisions for determining future use of the artwork after the artist's death. It is important to distinguish between copyrighted works and protected works under other laws. For example, literary works are protected by the U.S. Copyright Act and foreign authors have different rights depending on the country where they live. Patent law protects inventions that are new and useful. Trade secret law protects ideas that provide an advantage over others who do not protect them. The scope of copyright protection depends on two factors: first, whether the work is considered "original" (i.e., not copied from another source); second, whether the work is considered a "creative work" (i.e., not merely a description or list).