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Reference.org
Ischial tuberosity
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<p>The ischial tuberosity (or tuberosity of the ischium, tuber ischiadicum), also known colloquially as the sit bones or sitz bones, or as a pair the sitting bones, is a large <a href="/facts/Posterior_(anatomy)/IpJ1fq2f">posterior</a> <a href="/facts/Bone/kwJaR6IQ">bony</a> protuberance on the <a href="/facts/Superior_ramus_of_the_ischium/hfj1UDwx">superior ramus</a> of the <a href="/facts/Ischium/hfj1UDwx">ischium</a>. It marks the lateral boundary of the <a href="/facts/Pelvic_outlet/f3t1wUYM">pelvic outlet</a>. </p><p>When <a href="/facts/Sitting/KRDeAWL2">sitting</a>, the weight is frequently placed upon the ischial tuberosity. The <a href="/facts/Gluteus_maximus_muscle/uwzV7yN1">gluteus maximus</a> provides cover in the upright posture, but leaves it free in the seated position. The distance between a <a href="/facts/Cyclist/XH6EKjAF">cyclist</a>'s ischial tuberosities is one of the factors in the choice of a <a href="/facts/Bicycle_saddle/6voUgDBP">bicycle saddle</a>. </p>