The hip bone ( os coxae , innominate bone , pelvic bone or
coxal bone ) is a large flat bone, constricted in the center and expanded
above and below. In some vertebrates (including humans before puberty) it is
composed of three parts: the ilium, ischium, and the pubis.
The two hip bones join at the pubic symphysis and together with the sacrum and
coccyx (the pelvic part of the spine) comprise the skeletal component of the
pelvis – the pelvic girdle which surrounds the pelvic cavity. They are
connected to the sacrum, which is part of the axial skeleton, at the
sacroiliac joint.