The sacrococcygeal symphysis (sacrococcygeal articulation, articulation of the sacrum and coccyx) is an amphiarthrodial joint, formed between the oval surface at the apex of the sacrum, and the base of the coccyx. It is a slightly moveable joint which is frequently, partially or completely, obliterated in old age, homologous with the joints between the bodies of the vertebrae. [ http://en.wikipedia.org/wiki/Sacrococcygeal_symphysis ]