The study of joints is called arthrology. When two or more skeletal structures meet they form a joint or an articulation. In flat bones, such as those of the skull, it is the edges which form the joints; in irregularly shaped bones such as the tarsals it can be anywhere on their surfaces which articulate; and in long bones it is usually their extremities which form the joints, such as in the femur. Bones are united in three different ways. First, where movement is undesirable, the bones are united by fibrous tissue. Second, cartilage unites bones where a small degree of movement is necessary. Third, where a large amount of movement is called for, the bones are united by a tough membrane called the capsule; this in turn is lined by the synovial membrane.