The sacrum (/ˈsækrəm/ or /ˈseɪkrəm/; plural: sacra or sacrums; Latin os sacrum) in human anatomy is a large, triangular bone at the base of the spine, that forms by the fusing of sacral vertebrae S1–S5,between 18 and 30 years of age. The sacrum articulates (forms a joint) with four other bones. It is situated at the upper, back part of the pelvic cavity, where it is anatomically inserted between the two hip bones (ilium). The two lateral projections of the sacrum are called the alae (wings), and articulate with the ilium at the L-shaped sacroiliac joints. The upper part of the sacrum connects with the last lumbar vertebra, and its lower part with the coccyx (tailbone) via the sacral and coccygeal cornua. The sacrum has three different surfaces which are shaped to accommodate surrounding pelvic structures. Overall it is concave (curved upon itself). The base of the sacrum (the broadest and uppermost part) is tilted forward as the sacral promontory internally. The central part is curved outward toward the posterior, allowing greater room for the pelvic cavity. In all other quadrupedal vertebrates, the pelvic vertebrae undergo a similar developmental process to form a sacrum in the adult, even while the bony tail (caudal) vertebrae remain unfused. The number of sacral vertebrae varies slightly. A horse will fuse S1–S5, but a dog will fuse S1–S3. For example, the rat fuses four pelvic vertebrae between their lumbar and the caudal vertebrae of their tail. The Stegosaurus dinosaur had a greatly enlarged neural canal in the sacrum, characterized as a "posterior brain case".

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