The coccyx is formed by the fusion of three to five (usually four) coccygeal vertebrae to form a small triangular bone.  The coccygeal vertebrae are usually fused and consist of rudimentary vertebral bodies, possibly with traces of transverse processes and pedicles to the level of the second vertebra.  The second, third and fourth coccygeal vertebrae decrease in size as they descend.  

The coccyx descends antero-inferiorly, its upper end articulating with the sacral apex at the sacro-coccygeal joint.  Its orientation varies with its mobility, but generally the pelvic surface faces supero-anteriorly and the dorsal surface postero-inferiorly.  It gives attachment to the sacrotuberous and sacrospinous ligaments and coccygeus, levator ani and gluteus maximus muscles.

The base is formed by the upper surface of the first coccygeal vertebral body and exhibits an oval articular facet for the sacrococcygeal disc.  Postero-lateral to this facet are two processes referred to as the coccygeal cornua.  The coccygeal cornua project upwards to articulate with the sacral cornua.  Rudimentary transverse processes project supero-laterally from each side of the body.  They may articulate or fuse with the infero-lateral sacral angle to complete the fifth sacral foramina.  The interval between the body of the fifth sacral vertebra and the sacral and coccygeal cornua on each side forms an intervertebral foramen, transmitting the fifth sacral spinal nerve.

Ossification
Coccygeal segments each ossify from a single primary center with that of the first ossifying at birth and the others at various intervals until the age of 20 years.  The coccygeal cornua may ossify from separate centers soon after birth.  The segments gradually fuse; however, fusion between the first and second segments is commonly delayed until the 30th year.  In females, the coccyx often fuses with the sacrum later in life.