Ischial tuberosity

The tuberosity of the ischium (or ischial tuberosity, tuber ischiadicum in Latin, also known as the sitz bone, or as a pair the sitting bones) is a large swelling posteriorly on the superior ramus of the ischium. It marks the lateral boundary of the pelvic outlet.

When sitting, the weight is frequently placed upon the ischial tuberosity. The gluteus maximus covers it in the upright posture, but leaves it free in the seated position.