After the last lumbar vertebra is a mass of five smaller bones, which are naturally fused together forming a triangle. This bone mass is named the sacrum. Below the sacrum is the coccyx or tailbone made up of four little bones fused together.

The sacrum and coccyx do not look like the other vertebrae. The pairs of nerve roots originating from this area are responsible for the action of the pelvic organs and buttock muscles. The sacrum and tailbone could be