Hip bone each other at the pubic symphysis. Together with the sacrum and coccyx, the hip bones form the pelvis. Ilium (plural "ilia") is the uppermost and largest region. It makes up two fifths of the acetabulum. It is divisible into two parts: the body and the ala or wing of ilium; the separation is indicated on the top surface by a curved line, the arcuate line, and on the external surface by the margin of the acetabulum. The body of ilium forms the sacroiliac joint with the sacrum. The edge of the wing of ilium forms the S-shaped iliac crest which
Sacrum of the first piece of the coccyx, and this transmits the anterior division of the fifth sacral nerve. The thin lower half of the lateral surface gives attachment to the sacrotuberous and sacrospinous ligaments, to some fibers of the gluteus maximus at the back and to the coccygeus in the front. The sacrum articulates with four bones: Rotation of the sacrum superiorly and anteriorly whilst the coccyx moves posteriorly relative to the ilium is sometimes called "nutation" (from the Latin term nutatio which means "nodding") and the reverse, postero-inferior motion of the sacrum relative to the ilium whilst the coccyx