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Stable joints have articulating surfaces firmly united by connective tissue or cartilage, allowing little or no movement between bones. Freely moving joints enclose articulating surfaces in a space filled with lubricating fluid, permitting greater mobility. This inverse relationship means that joints providing the most movement are the least stable, while stable joints restrict mobility.

Joints provide protection, stability, and flexibility to the skeletal system. Skull joints interlock bones protecting the brain, intervertebral joints stabilize the spine, and shoulder joints enable arm movement. By connecting bones and allowing controlled motion or providing rigid support, joints enable both structural integrity and functional movement throughout the body.

Skull joints are held together by fibrous connective tissue and remain immovable to protect the brain from damage. Shoulder joints, conversely, have articulating surfaces enclosed in lubricating fluid, allowing wide ranges of motion for arm movement. Joint anatomy directly determines its ability to move, so protective structures require stability while functional areas require mobility.

Cartilage and fibrous connective tissue unite bones at stable joints, restricting movement and providing weight-bearing support. For example, the tibia and fibula are tightly united by connective tissue for standing stability, while vertebral joints use cartilage to permit only small movements. These materials create firm unions that prioritize structural integrity over mobility.

Vertebral joints are united by cartilage, which permits only limited movements between adjacent vertebrae. However, when these small individual movements combine, they provide sufficient flexibility for the body to twist and bend in multiple directions. This design demonstrates how restricted individual joint mobility can create overall spinal flexibility while maintaining stability.

The appendicular skeleton requires extensive mobility for activities like walking, running, and jumping, so it contains mostly freely moving joints with lubricating fluid-filled spaces. The axial skeleton prioritizes protection and support, featuring mainly immovable and slightly movable joints. This distribution reflects how joint location and function determine skeletal structure.

Structural classification depends on joint anatomy, such as whether bones are united by fibrous tissue, cartilage, or enclosed in fluid-filled spaces. Functional classification is based on the types of movements joints permit. These classifications overlap because a joint's anatomical structure directly determines its movement capabilities, making anatomy the foundation for both systems.