Components are the units of deployment — JARs, DLLs, shared libraries. Martin traces their history from early relocatable binaries to modern package managers, establishing the foundation for the component cohesion and coupling principles that follow.
Three principles govern which classes belong in which component: REP (Reuse/Release Equivalence), CCP (Common Closure), and CRP (Common Reuse). They form a tension triangle — understanding the trade-offs helps you choose the right grouping strategy for your project's stage.
Three principles govern dependencies between components: ADP (no dependency cycles), SDP (depend in the direction of stability), and SAP (stable components should be abstract). These principles shape large-scale architecture and determine which components can evolve independently.