Mechanics are one of the pillars of gameplay, enabled by the underlying implementation of the game. In particular, self-amending mechanics are mechanics that change themselves dynamically and are a common source of duplicate and coupled code because they occur in multiple situations using specific interactions. The Rulebook is an architectural pattern that generalizes how developers deal with these issues, based on a careful research process including a systematic literature review, semi-structured interviews with professional developers, and quasi-experiments. The pattern codifies changes to the game state as “effect” objects, which it matches against a dynamic pool of rules. Each rule may amend, resolve, or chain effects. This design prevents the rest of the game from becoming coupled to the specific interactions of these mechanics while also promoting an extensible and flexible structure for self-amendment. This paper details the Rulebook pattern and presents a case study demonstrating its design process in three different implementations of open-source jam games. Together with the typification of self-amending mechanics, this article formalizes a novel, state-of-the-art toolset for architecting games.