Continuing down the posterior side, the ischial tuberosity is a thick, rough-surfaced prominence below the lesser sciatic notch.

From its posterior border, there extends backward a thin and pointed triangular eminence, more or less elongated in different subjects, the ischial spine, origin of the gemellus superior muscle.

Below, it is bounded by a sharp ridge that provides attachment to a falciform prolongation of the sacrotuberous ligament, and, more anteriorly, gives origin to the transverse perineal and ischiocavernosus muscles.

Its medial border is thick, rough, slightly everted, forms part of the outlet of the pelvis, and presents two ridges and an intervening space.

Avulsion fractures occur where muscles are attached: anterior superior and inferior iliac spines, ischial tuberosities, and ischiopubic rami.

Adoption of ischium into English-language medical literature dates back to c. 1640; the Latin term derives from Greek ἰσχίον iskhion meaning "hip joint".

The division of the acetabulum into ischium (ἰσχίον) and ilium (λαγών, os lagonicum) is due to Galen, De ossibus.

[5] The clade Dinosauria is divided into the Saurischia and Ornithischia based on hip structure, including importantly that of the ischium.

The acetabulum, which can be thought of as a "hip-socket", is a cup-shaped opening on each side of the pelvic girdle formed where the ischium, ilium, and pubis all meet, and into which the head of the femur inserts.

The orientation and position of the acetabulum is one of the main morphological traits that caused dinosaurs to walk in an upright posture with their legs directly underneath their bodies.