What is the merger doctrine?

Ideas are not given any protection under copyright law - only creative expression of an idea is capable of being protected. However, occasionally an idea can only be expressed in one way, or in an extremely limited number of ways. For example, there is only one way to express the rules to a contest (by listing them out). When the way an idea can be expressed is so limited, it is said that the idea and the expression “merge” and the expression is therefore not entitled to copyright protection. So the main focus of this issue is how many ways an idea can effectively be expressed.