The ischium forms the postero-inferior part of the hip bone. The superior part of the body of the ischium fuses with the pubis and ilium, forming the postero-inferior aspect of the acetabulum. The ramus of the ischium joins the inferior ramus of the pubis to form a bar of bone, the ischiopubic ramus, which constitutes the inferomedial boundary of the obturator foramen. The posterior border of the ischium forms the inferior margin of a deep indentation called the greater sciatic notch. The large, triangular ischial spine at the inferior margin of this notch provides ligamentous attachment. This sharp demarcation separates the greater sciatic notch from a more inferior, smaller, rounded, and smooth-surfaced indentation, the lesser sciatic notch. The lesser sciatic notch serves as a trochlea or pulley for a muscle that emerges from the bony pelvis. The rough bony projection at the junction of the inferior end of the body of the ischium and its ramus is the large ischial tuberosity. The body's weight rests on this tuberosity when sitting, and it provides the proximal, tendinous attachment of posterior thigh muscles.