Sacrum

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) 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.