The joint between the sacrum and the coccyx, the sacrococcygeal symphysis, is strengthened by a series of ligaments.

http://en.wikipedia.org/wiki/Pelvis

The joint between the sacrum and the coccyx, the sacrococcygeal symphysis, is strengthened by a series of ligaments. The anterior sacrococcygeal ligament is an extension of the anterior longitudinal ligament (ALL) that run down the anterior side of the vertebral bodies. Its irregular fibers blend with the periosteum. The posterior sacrococcygeal ligament has a deep and a superficial part, the former is a flat band corresponding to the posterior longitudinal ligament (PLL) and the latter corresponds to the ligamenta flava. Several other ligaments complete the foramen of the last sacral nerve.