The pelvic girdle consists of the two coxal bones (= pelvic or innominate or hip bones). These articulate with the sacrum behind at the sacroiliac joint, and with each other in front at the pubic symphysis. Together with the sacrum the pelvic bones enclose a cavity, the pelvis, through which pass the terminal portions of both the digestive and the urinogenital systems.

The form of the pelvis reflects the functions of the lower limbs. Note the triangular shape of the sacrum which is wedged between the pelvic bones, thus transmitting weight from the vertebral column laterally to the head of the femur. The latter bone articulates with the pelvis in a deep cup shaped socket, the acetabulum. This cup faces down and forward. Its superior and posterior walls are stronger and heavier than those on the opposite sides, reinforcing the joint for the transmission of weight (from above) and for the propulsive thrust of the legs in walking (from in front).

In the child the pelvic girdle consists of 3 pairs of separate bones, a superior element (the ilium) and two inferior elements, the ischium posteriorly and the pubis anteriorly. All three bones meet within the acetabulum. These bones fuse during adolescence to form the single coxal bone on either side characteristic of the adult.

Identify the ilium, ischium and pubis, and the acetabulum. On the ilium locate the iliac crest, the bony ridge palpable when you place your hands on your hips. Note the large and rather rough surface for articulation with the sacrum (sacroiliac joint). The ischium is the strongest component of the pelvic bone. Note the large ischial tuberosity on which the weight of the body rests when sitting, thus resting the legs. The main feature of the pubis is the articular surface of the pubic symphysis. A large opening, the obturator foramen is formed between the ischium and pubis below the acetabulum. Its chief function is to lighten the weight of the pelvis.

The shape of the pelvis shows distinct sexual differences.
In general the female pelvis is lighter, shallower and wider than that of the male, and the pelvic outlet is larger, and oval rather than heart shaped. There are also sexual differences in the shape of the sacrum: in the female it is wider and shorter.