The idea-expression divide is a concept in copyright law which states that copyright does not protect ideas, information or function, but only the fixed expression of ideas. Only the way in which something has been expressed is protectable by copyright.

For example, if a book is written describing a new way to organize books in a library, a reader can freely use that method without being sued, but what is written in the book, the original expression of the idea, may not be copied. One might be able to obtain a patent for the method, but that is a different body of law. If there is art on the cover of the book, that art may also be copyrighted. If the book lists only facts, one might say that it is not original, but if the facts are selected and arranged in an original manner, the book may be copyrighted.

In the English decision of Donoghue v. Allied Newspapers Ltd (1938) Ch 106, the court argued that "the person who has clothed the idea in form, whether by means of a picture, a play or a book" owns the copyright. Even more eloquently, Latham CJ in the Australian decision of Victoria Park Racing and Recreation Grounds Co. Ltd v. Taylor (1937) 58 CLR 479 and 498 argued that if you are the first person to announce that a man has fallen off a bus, you cannot use the law of copyright to stop other people from announcing that fact.

Some courts have recognized that there are particular ideas that can be expressed intelligibly only in one or a limited number of ways. Therefore even the expression in these circumstances is unprotected, or extremely limited to verbatim copying only. In the United States this is known as the merger doctrine, because the expression is considered to be inextricably merged with the idea. U.S. courts are divided on whether merger constitutes a defense to infringement or prevents copyrightability in the first place, but it is often pleaded as an affirmative defense to infringement.