The cranial extremity of sacrum is termed the base:
On the dorsal part of the base of sacrum, are the cranial articular processes that articulate with the caudal articular processes of the last lumbar vertebra.
The ventral border of the base, the promontory, projects together with the last intervertebral disc into the pelvic inlet.
The lateral parts of the base, composed by the fused transverse processes of sacral vertebrae (mainly the first), expand laterally to form the wings of sacrum. Each wing carries on his dorsal part an oval articular surface covered with cartilage, termed auricular surface , for articulation with the ilium. The sacral tuberosity is the roughed surface, dorsal to the auricular surface, for attachment of ligaments.