The base of the vertebral column is composed of (number) fused vertebrae and is called the sacrum.

The base of the vertebral column is composed of five fused vertebrae and is called the sacrum. The sacrum is a triangular-shaped bone that sits at the base of the spine and connects to the pelvis. It plays a crucial role in supporting the weight of the upper body when a person is standing or walking. The five vertebrae that form the sacrum are numbered S1 to S5. During childhood, these vertebrae are individual bones, but they gradually fuse together during late adolescence into early adulthood, providing a strong structure that also helps form the pelvic cavity.