Abstract The sacrum is a large triangular bone formed from the five fused sacral vertebrae (Figure 1). Two rows of foramina on its dorsal and pelvic surfaces divide it into a median portion, the body and two lateral masses. The paired pelvic and dorsal sacral foramina communicate by intervertebral foramina with the central sacral canal and convey the ventral and dorsal rami of the sacral nerves. The base, directed upwards and forwards is formed from the upper surface of the first sacral vertebra. Lateral to this, the lateral masses of the sacrum are at their widest and form the prominent ala. The protruding central part of the superior surface is known as the promontory.