idea-expression-divide

The line copyright walks: you cannot own an idea, but you can own the particular way you expressed it — and where the idea and the expression merge into one, nothing is left to protect.

Codified in TRIPS Article 9(2) and followed in 164+ countries, the doctrine holds that copyright protects expression, not the ideas, procedures, or facts behind it. The sharp edge is the merger doctrine: when an idea can be expressed in only a few ways, those expressions merge with the idea and become unprotectable — the words are forced, the expression is the idea's only coat. A coined technical term that names a real concept sits close to the fact side (it is a needed word); its first definition sits closer to the expression side (a particular arrangement of words). But where the definition is forced — only one or two obvious phrasings — it merges too. The border is a spectrum, not a line; as Judge Learned Hand put it, "nobody has ever been able to fix that boundary, and nobody ever can."