1 — Coccyx [coccygeal vertebrae CoI-CoIV]; 2 — Coccygeal cornu; 3 — Posterior sacral foramina; 4 — Lateral part; 5 — Sacral canal; 6 — Superior articular process; 7 — Sacral tuberosity; 8 — Auricular surface; 9 — Lateral sacral crest; 10 — Median sacral crest; 11 — Intermediate sacral crest; 12 — Sacral hiatus; 13 — Sacral cornu; Sacral horn; 14 — Sacrococcygeal joint; 15 — Dorsal surface; 16 — Base of sacrum;17 — Promontory; 18 — Pelvic surface