Coccyx

The 'coccyx\', commonly referred to as the tailbone, is the final segment of the human vertebral column, of three to five (usually four) fused vertebrae (the coccygeal vertebrae), below the sacrum. It is attached to the sacrum in a fibrocartilaginous joint, which permits limited movement between them.

The coccyx is regarded as vestigial in humans, meaning it no longer serves major functions it did in ancestor species of humans. (Those included supporting a tail and accomodating its nerves.) It does provide an attachment for muscles, such as the gluteus maximus, and also serves as something of a shock absorber when the person sits down.

The coccygeal bones fuse as a person ages.

Sitting down too quickly on a hard surface can cause painful injury to them.