The twelfth rib is a short, slender curved bone with a head and shaft, but no neck, tubercle or angle.  The head has a single facet for articulation with the middle of the twelfth thoracic vertebral body.  The tapering shaft has smooth internal and external surfaces, and rounded superior and sharper inferior borders.  The internal border faces slightly superiorly.  The free anterior end is tipped with cartilage.

Ossification
The twelfth rib ossifies from primary and secondary centers.  The primary ossification center for the shaft appears during the eighth week in-utero.  The secondary ossification center for the head appears during puberty and fuses with the shaft after the age of 20 years.