A joint (or articulation) is the point where two or more bones meet, with muscles acting on them to produce movement — though not every joint is actually movable.

The chapter classifies joints in two independent ways. The FUNCTIONAL classification looks at how much movement a joint allows: synarthroses are immovable joints, amphiarthroses allow slight movement, and diarthroses are freely movable. Freely movable joints are concentrated in the limbs, where mobility matters most, while immovable/slightly movable joints are concentrated in the axial skeleton, where protecting internal organs and providing a firm, stable attachment matters more than mobility.

The STRUCTURAL classification looks at what actually separates the bones at the joint: fibrous joints (bones united by fibrous tissue — generally immovable, further split into sutures of the skull, syndesmoses like the radius-ulna membrane, and gomphoses like a tooth in its socket), cartilaginous joints (bones united by cartilage — only very slight movement, e.g. between the ribs and the sternum), and synovial joints (freely movable, cushioned by synovial fluid, built from an articular cartilage, articular capsule, joint cavity and reinforcing ligament). …