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The sacrum (sacral bone) is a large triangular-shaped bone located at the distal aspect and base of the spine. It is composed of five fused sacral vertebrae that begin to merge at puberty. Usually, the fusion ends around 30 years. The number of sacral vertebrae can vary among people. The sacrum not only participates in spine formation but also contributes to the formation of the pelvic skeleton. Therefore, it links the spine with the pelvis.