The remnant of the tail in the human skeleton is the coccyx. Commonly called the tailbone, it sits at the very end of the vertebral column, below the sacrum.
The coccyx consists of three to five small coccygeal vertebrae, which are often partly or completely fused. In our tailless primate ancestors, the tail became reduced over evolutionary time, leaving this compact structure. Human embryos briefly develop a tail-like extension early in development, which is later absorbed as the spine and pelvis form.
Although the coccyx is vestigial as a tail, it is not entirely useless. It provides attachment points for pelvic-floor muscles, tendons, and ligaments, and helps support the body while sitting. Injury to it can cause coccydynia, or persistent tailbone pain.
A common mix-up is confusing the coccyx with the sacrum. The sacrum is the larger, triangular bone above it that connects the spine to the pelvis; the coccyx is the small terminal bone beneath the sacrum.