- The sacrum and coccyx are part of the spine and are located in the pelvis.
- The sacrum articulates with:
- The sacrum is composed of five vertebral elements (S1-S5) that do not fully form and that fuse into one bone.
- The sacrum is an upside-down triangle, with the base of the sacrum at the top (superior end) and the apex of the sacrum at the bottom (inferior end).
- The coccyx is usually said to be composed of two-four vertebral elements that do not fully form.
- The coccyx does not ossify until later in adulthood.
- The coccyx is considered to be the evolutionary remnant of a tail.
- The pelvis is a transitional body part. It has both axial body bones (sacrum and coccyx) and appendicular body bones (pelvic bones) within it.