CoccyxThe tailbone, made of four very small vertebrae fused together. It joins the lower tip of the sacrum.
BaseThe broad upper end, which bears weight. Its oval surface is the top of the first sacral vertebra and meets the last lumbar vertebra.
Sacral promontoryThe front lip of the base of the sacrum. Here it meets the last lumbar vertebra.
Superior articular processOne on each side of the opening of the sacral canal. They join the inferior articular processes of the fifth lumbar vertebra.
Anterior sacral foraminaFour pairs of openings on the front surface. The front branches of the sacral spinal nerves leave through them.
Transverse ridgesFour ridges across the front of the sacrum. They mark where the five sacral vertebrae fused.
ApexThe lower tip of the sacrum, which does not bear weight. It meets the coccyx.
Back view 6
Sacral canalA bony tunnel through the sacrum, the continuation of the vertebral canal. It carries the sacral nerves.
Median sacral crestA ridge down the midline of the back surface. It is what is left of the fused spinous processes.
Posterior sacral foraminaFour pairs of openings on the back surface. The back branches of the sacral spinal nerves leave through them.
Lateral sacral crestA row of bumps outside the posterior foramina on each side. They are the fused transverse processes of the sacral vertebrae.
Auricular surfaceThe rough, ear-shaped surface on each side. It meets the ilium of the hip bone at the sacroiliac joint.
Sacral hiatusThe opening at the lower end of the sacral canal. It lies near the tip of the sacrum.
How this list was compiled: Every name and detail checked on 2026-09-30 against OpenStax Anatomy and Physiology 2e, section 7.3 The Vertebral Column (the sacrum and coccyx: base, apex, transverse ridges, median and lateral sacral crests, sacral promontory, auricular surface, sacral canal, sacral hiatus, anterior and posterior sacral foramina, superior articular process, coccyx), https://openstax.org/books/anatomy-and-physiology-2e/pages/7-3-the-vertebral-column, and against Wikipedia, Sacrum and Coccyx, text only. Licence: own work
The drawing: Drawn for this site on 2026-09-30 from plain geometry, after the layout of Gray's Anatomy (1918, public domain) figures 95 and 96; nothing traced. Licence: own work
How it works
The front view is the side of the sacrum that faces into the pelvis. The back view has the crests, the sacral canal and the sacral hiatus. The coccyx sits below in both.
Read the full guide
Front first, then back
On the front: the coccyx, the base and its front lip, a superior articular process, the holes and ridges across the middle, and the apex. On the back: the canal at the top, the crest down the midline, the holes beside it, the crest outside them, the ear-shaped side surface and the hiatus near the bottom.
Median is the single crest on the midline; lateral is the pair out to the sides.
Base is the wide top and apex is the narrow bottom.
Questions and answers
What are the parts of the sacrum?
A wide base with the sacral promontory as its front lip, a narrow apex, and transverse ridges and anterior foramina in front. Behind are the median and lateral sacral crests, posterior foramina, sacral canal and sacral hiatus. Each side has an auricular surface.
How many bones make up the sacrum?
Five sacral vertebrae, fused into one triangular bone. The four transverse ridges across the front mark where they joined.
What is the difference between the sacrum and the coccyx?
The sacrum is the large triangular bone at the bottom of the spine, made of five fused vertebrae. The coccyx, or tailbone, is the small bone below it, made of four very small fused vertebrae.
What passes through the sacral foramina?
Branches of the sacral spinal nerves. The foramina open into the sacral canal: the anterior branches leave through the openings on the front and the posterior branches through those on the back.