When an idea can only be expressed in a few ways (the two are linked or “merged”), then expression is so limited that it cannot be copyrighted. This is known as the merger doctrine.
“Under the merger doctrine, courts will not protect a copyrighted work from infringement if the idea underlying the work can be expressed only in one way, lest there be a monopoly on the underlying idea. In such an instance, it is said that the work’s idea and expression ‘merge.’”
— Joshua Ets-Hokin v. Skyy Spirits Inc., 2000