The sacrum is a bone located at the base of the spine that consists of five vertebrae. These vertebrae are unfused in children, but by the age of 26 they will be fused together. It is triangular in shape and connects the final lumbar vertebra with the coccyx, also known colloquially 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 different in women than in men, which is known as sexual dimorphism. In men, it is narrower and longer. The lower half is at a lesser angle than in women. As a result, the pelvic cavity is generally narrower. This is what gives women the prominent hourglass shape, which allows them to bear children. 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.