Posterior superior iliac spine (PSIS)

Spina iliaca posterior superior

How to find it: The two dimples in the lower back (the dimples of Venus) lie right over it; under the skin you feel a firm, round knob.
What you use it for: Reference for the SI joints and for pelvic position; the joint itself lies directly medial and deep to it.
On the bone: Hip bone
Body region: Pelvis