  The term 'double-jointed' is a little misleading. People who can do strange things with their body parts don't necessarily have extra joints, but rather joints which rotate or allow movement that is more extreme than most people can manage.   A dictionary definition of double-jointedness makes it clear: it is " having unusually flexible joints that can bend in unusual ways or to abnormally great extent." Many people have a finger or arm that they can bend in an unusual way. This is quite common. Other people have more extreme flexibility, particularly in their spine, and can contort their bodies into strange positions. A famous Cirque de Soleil contortionist, when challenged for a description of what she could do with her body, said "We are four little girls who fold". Occasionally you will hear the term 'hypermobility' used to describe this condition. But this too is misleading, because the prefix 'hyper' in medicine means 'too much', and probably should be reserved for ligament injuries or other actual disabilities. People with unusual joint flexibility are not overextending their muscles or joints ... they are allowing them to function to their full potential.   Joints are the junction between two bones. The shape of the bones where they fit together determines the natural flexibility of the joint. A shallow socket allows more movement in both directions than a deep one. Sometimes, as in the ball-and-socket joints of the limbs, an especially shallow socket may allow a person to 'dislocate' one bone completely from the socket of the other, and then move it back. But more often, extreme flexibility is just the result of a wider range of movement between the bones, caused by a shallow socket, smoother than normal bone ends, and ligaments which are extra stretchy. The cartilage that acts as padding between bones is made from a protein called collagen, which can vary in its suppleness. If your cartilage is especially flexible, the joint will allow more movement. This is visibly the case with children, whose cartilage is very flexible, especially in the spine. In children, often the discs of cartilage separating the spinal discs (bone) are larger than the spinal disks themselves! But with age, the cartilage and bones harden. This is why children can do gymnastics and their parents can't! When cartilage gets old and worn down with old age, you may start to get joint pain as the bones rub together.  The photo on the left shows the spine of a contortionist who is bending over backwards ... like the girl above. Notice how the vertebrae don't scrape together, but the muscles and ligaments in the spine allow the vertebrae to stretch farther apart than usual. The vertebrae are not separated completely, even when extended this way; they are held together with ligaments and muscles. This type of flexibility needs to be exercised often, or the wide range of motion will be lost. Interestingly, sports medicine researchers have discovered that some weight training and exercise programs used to build up muscle mass actually reduce the flexibility of the muscles (you have more muscle mass, and bigger muscles, but they won't stretch as far). This may not help the athlete at all. Here are some more pictures of people who have more than the average range of motion in their joints.     
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