The sacrum is a shield-shaped bony structure that is located at the base of the lumbar vertebrae and that is connected to the pelvis. The sacrum forms the posterior pelvic wall and strengthens and stabilizes the pelvis. Joined at the very end of the sacrum are two to four tiny, partially fused vertebrae known as the coccyx or "tail bone". The coccyx provides slight support for the pelvic organs but actually is a bone of little use.Update Date: 1/24/2007 Updated by: Stuart Bentley-Hibbert, M.D., Ph.D., Department of Radiology, Weill Cornell Medical Center, New York, NY. Review provided by VeriMed Healthcare Network.