Sacrum, coccyx and lower part of spine, computer artwork. The sacrum is a large, triangular bone at the base of the spinal column made up of five fused vertebrae. It sits like a wedge between the two iliac bones of the pelvis (not seen). Below the sacrum is the coccyx, or tailbone, which is also made up of four or five fused vertebrae. Above the sacrum is the lumbar region of the spine.