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 the direction of rotation of the superior portion of the sacrum opposite the side of the superior axis pole, and the last letter pertains to the pole. English "sacrum" was introduced as a technical term in anatomy in the mid-18th century, as a shortening of the Late Latin name "os sacrum" "sacred bone", itself a translation of Greek ἱερόν ὀστέον, the term found in the writings of Galen. Prior to the adoption of "sacrum", the bone was also called "holy bone" in English, paralleling German "heiliges Bein" or "Heiligenbein" (alongside "Kreuzbein") and Dutch "heiligbeen". The origin of Galen's term is