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The sacrum is the triangular bone just below the lumbar vertebrae. The sacrum has five segments fused together into one large bone. In the months before birth, these vertebrae grow together into one bone that forms the base of the spine and the center of the pelvis.
This triangular bone fits between the two halves of the pelvis, connecting the spine to the lower half of the body.
Nerves that leave the spine in this area help control the bowels and bladder and provide sensation to the crotch area. The two dimples in most everyone's back are where the sacrum joins the hipbones (sacroiliac joint).
The point where the lower back joins the sacral region (the L5-S1 joint) can develop pain due to injury. This area receives a great deal of stress and twisting during activities, such as sports or sitting for long periods.