The sacrum is a large wedge shaped vertebra at the inferior end of the spine. It forms the solid base of the spinal column where it intersects with the hip bones to form the pelvis. The sacrum is a very strong bone that supports the weight of the upper body as it is spread across the pelvis and into the legs.
The sacral region (sacrum) is at the bottom of the spine and lies between the fifth segment of the lumbar spine (L5) and the coccyx (tailbone). The sacrum is a triangular-shaped bone and consists of five segments (S1-S5) that are fused together.
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.This diagram with labels depicts and explains the details of Sacral Anatomy.