The sacrum is the name of the bone located at the base of the spine that consists of five fused vertebrae. These vertebrae are unfused in children, but by the early to mid twenties, they will be fused together. It is triangular in shape and connects the final lumbar vertebra with the coccyx, which is commonly referred to as the tailbone.

The sacrum is curved, which allows more room in the pelvic cavity for various organs. The sacrum is a little unusual in that it is differently shaped in males and females, which is known as sexual dimorphism. In males, it is narrower and longer. The lower half is at a smaller angle than in females. As a result, the male pelvic cavity is generally narrower. Females have a wider pelvic cavity in order to allow for pregnancy and childbirth, as well as to house the reproductive organs.

Occasionally, the sacrum may consist of four pieces, rather than the usual five. This is due to the lack of fusion of the first and second sacral vertebrae. In addition, the angle at which the sacrum curves can vary widely from person to person (even of the same sex).