Nine fused vertebrae form the sacrum and the coccyx also known as the tail bone. Each vertebra is designated with a letter indicating the topographic region (C, T, L and S) and the number of order. For example: C6 is the sixth cervical vertebra; T8 is the eighth thoracic vertebra. The first two cervical vertebrae are the only vertebrae with specific names; C1 is called the atlas and C2 is called the axis. The fifth lumbar vertebra (L5) may be congenitally partially or completely fused with the upper sacrum. This is called sacralization of L5. Occasionally, the first sacral segment (S1) may remain a separate bone, resulting in six lumbar vertebrae, called lumbarization of S1.