Answer
One definition of a joint or articulation is that a joint is a point where two bones meet. The bones may be moveable, as is the case with the bones of the elbow joint; they may immovable, as is the case with the cranial bones . Also, they may be moderately moveable, as is the case with vertebrae of the back bone.
The knee joint includes three bones, humerus, tibia and patella; the so-called shoulder joint is really more than one joint, and the so-called ankle joint is really three different joints. It should also be remembered that there are really four bones involved in the hip joint: the hip bone which is really three fused bones, ilium, ischium and pubis - acetabulum, and the femur.
Work Step by Step
Not all joints are moveable. Some joints start out with varying degrees of motility, but ossify over time and become immovable. Such is the case of the left and right mandibles of infants which progressively become one bone with development.
Joints not only facilitate movement, they also provide protection of soft tissues, help maintain posture ,and offer support of body parts.
