Joints or articulations are areas in the skeleton where two bones meet. While the rigidity of bones give structure and support to the human body, joints contribute to the protection, stability, and flexibility provided by the skeletal system. For example, skull joints interlock the bones that protect the brain, intervertebral joints provide stability to the spine, and shoulder joints allow the movement of the arms. The axial skeleton consists mainly of immovable and slightly movable joints, while the appendicular skeleton contains the majority of freely moving joints. Joints can be classified structurally, depending on their anatomy, or functionally, based on their movements. However, both these classifications overlap because a joint's anatomy determines its ability to move.