The merger doctrine is a practice in United States copyright law which says that if an idea and the way to express it are so intricately tied that the ways of expression have little possible variation, there will not be copyright infringement, lest the copyright prevent others from expressing the same idea. The overall principle is that of the idea-expression divide, where you can copyright an expression but not an idea.
One case involving this was Ets-Hokin v. Skyy Spirits Inc., 225 F.3d 1068 (9th Cir. 2000) [1], which found that the whole vodka bottle in that case was driven by function, not copyrightable and could not be infringed for that reason. While the label might have been copyrightable, it was ruled to be incidental to the use and not significant.