The prominent and important structures in this area are all musculoskeletal. The bony sacrum forms the posterior part of the bony pelvic ring and is the distal continuation of the vertebral column. Formed by the fusion of the five sacral vertebrae (the number of vertebrae that fuse to form the sacrum varies from four to six, but is commonly five), the sacrum is a complexly curved and heavy bone that is shield shaped when viewed from behind. The posterior surface is roughened and has two paramedian crests—the lateral sacral and intermediate crest—which, with the prominent midline median sacral crest, form points of attachment for fascial and aponeurotic structures. Four broad posterior sacral foramina between the five fused vertebrae are points of ingress and egress for the dorsal rami of the sacral spinal nerves. Viewed from the side, a prominent anterior concavity, commonly termed the hollow of the sacrum, is obvious. This forms a space in which lie the rectum, muscles of the pelvic diaphragm, neurovascular structures, and a variable amount of fat. At the top of this concavity, the sacral promontory (located at the point of articulation of the body of the lowest lumbar vertebra with the sacrum) forms an easily palpable bony landmark for the surgeon operating within the pelvis. The sacrum is shorter and wider in the female pelvis than in the male pelvis, contributing to the wider, rounder gynecoid shape that is designed to accommodate the head of a full-term infant at the time of delivery.

The coccyx is composed of three to five remaining vertebrae (commonly, four). These small, nubbin-like, rudimentary vertebrae articulate with the sacrum. Only the first coccygeal vertebra possesses identifiable transverse processes and homologues of pedicles (coccygeal cornua). No vertebral canal is present. The mobility of the coccygeal vertebrae varies considerably from individual to individual, and the terminal three coccygeal vertebrae are commonly fused.