The =os coxæ= (or os innominatum) forms the skeleton of the hip or
haunch, and is the largest of the flat bones. It consists primarily of
three parts, the =ilium=, =ischium=, and =pubis=, which meet to form the
=acetabulum=, a large cotyloid cavity for articulation with the head of
the femur. These parts are fused at about one year of age, but it is
convenient to describe them separately.