Merger Doctrine is a principle of copyright law which says when there is only one or limited number of ways to express an idea, copyright law will not protect the expression because it has "merged" with the idea. When the idea and expression are very difficult to separate, they are said to merge. The rationale arose in the case Baker v. Selden, 101 U.S. 99 (U.S. 1880). It was later applied to Morrissey v. Procter & Gamble Co., 379 F.2d 675 (1st Cir. Mass. 1967), wherein it came to known as the Merger Doctrine.