This chapter explores how game mechanics function as the structural foundation of digital gameful compositions (DGCs). Building on the frameworks of game design and interactive music, it examines how mechanics—the rules, actions, and systems that govern interaction—can serve as musical scores, instruments, and performance spaces. The chapter begins by analysing how mechanics communicate musical information, drawing parallels between traditional notation, animated digital scores, and game-based representations of time, pitch, loudness, and timbre. It then considers how mechanics can generate sound directly, functioning as instruments that respond to player input through complex mappings. The discussion extends to virtual spaces as meta-venues for performance, where spatial design shapes musical experience and performer agency. Finally, the chapter addresses how rules and affordances guide player behaviour, and how simple mechanics can yield emergent musical complexity through strategic interaction. Drawing on examples from contemporary DGCs, the chapter demonstrates how composers use game mechanics not only to structure musical behaviour but also to create dynamic, cybernetic systems that foreground performer agency.