Intergluteal cleft

The intergluteal cleft, also known medically by various synonyms and colloquially as the "butt crack" or "ass crack", is the groove between the buttocks that runs from just below the sacrum to the perineum, so named because it forms the visible border between the external rounded protrusions of the gluteus maximus muscles. The intergluteal cleft is located superior to the anus.

Ingrown hair can obstruct hair follicles in the intergluteal cleft, exciting a foreign body reaction, leading to formation of a pilonidal cyst. Secondary tracks may open onto the skin, allowing the cyst to drain a foul smelling discharge, which is known as a pilonidal sinus.