Inferior to the fifth lumbar vertebra (L5) are the sacrum and coccyx bones. The large, triangular-shaped sacrum (os sacrum) is composed of five vertebrae (S1 - S5) that fuse during early adulthood (18 - 30 years). The smaller coccyx (os coccygis) articulates with S5 and is also known as the tailbone. It consists of three to five vertebrae (C1 - C5) that are also usually fused.