A sacrum is a large, triangular bone at the base of the spine that forms part of the pelvis. It results from the fusion of five sacral vertebrae (S1–S5) and serves as a key structural foundation that supports the upper body and distributes weight through the pelvis into the legs. The sacrum has four joints with the pelvic bones (the sacroiliac joints) and connects upward to the last lumbar vertebra (L5) and downward to the coccyx (tailbone). It also contains a anterior (pelvic) and posterior surface with ridges and foramina for nerve and vessel passage. The sacrum’s shape and fusion enable it to bear substantial loads and contribute to pelvic stability and mobility.