The place where bones connect to each other is called the osseous linkage, and there are two types of links: direct and indirect. The direct connection is the connection between the bones of the skull and the trunk. Direct joints are usually made by connecting the bones together with cartilage or fibers, with no gaps in between, and are generally immobile, or can only be moved to a small degree. For example, the cranial bones are connected by fibers; the vertebrae of the spine are connected by cartilage, which allows the spine to be flexible and elastic; and the pelvis is also formed by direct connection, and many of the pelvic bones will eventually ossify and fuse in the joints. Indirect joints, also known as articular structures, are generally formed by multiple structures connecting bone to bone. For example, the knee joint connects the femur to the tibia through muscles, ligaments, joint capsules, and articular cartilage.