The six types of joint are all synovial joints. They allow movement while protecting the bones from damage when they move against each other. Bone ends are covered with smooth, slippery cartilage, which helps reduce friction. The space between the bones is filled with a liquid called synovial fluid. This lubricates the joint and provides a liquid cushion between the bones. Non-synovial joints, such as the sutures of the skull, do not move.
In most free-moving joints, the ends of bones do not touch each other but are held together by tough straps called ligaments. Although they held the joint together, ligaments are elastic enough to allow you to move. The ends of bones in free-moving joints are covered with smooth, slipper cartilage. The space between bone ends also contains some oily liquid. Together the cartilage and liquid allow the bone ends to slide over each other so that the joint can move smoothly.
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