The coccyx (os coccygis) is a small triangular bone. It is formed from three or four vertebrae fused together. It articulates proximally with the apex of the sacrum. Coccygeal vertebrae are remnants of the tail and rudimentary vertebrae, fused together to form a single triangular bone, the coccyx. The number of coccygeal vertebrae varies from 3 to 5. The coccyx has only rudimentary vertebral elements of vertebra but these include:

Coccygeal horn (cornu coccygeum) or coccygeal cornu is a horn-shaped outgrowth of the rudimentary articular processes of the coccyx that articulate with the cornu of the sacrum.

Transverse process (processus transversus) is small and only at the first coccygeal vertebra.

Base of coccyx (basis ossis coccygei) presents an oval surface for articulation with the sacrum.

Apex of coccyx (apex ossis coccygei) usually has rounded  surface but it may be bifid. The apex is sometimes deflected  to one or other side.

Clinical comments

Coccygodynia is a pain in or around the region of the coccyx. It is 5 times more common in women than in men. The most common are direct falls resulting in direct injury of the coccyx region. The result is an injury or partial dislocation of the sacrococcygeal junction that causes abnormal movement of the coccyx. In the majority of cases, the pain occurs only in the sitting position.