Sacrum

Its upper part connects with the last lumbar vertebra, and bottom part with the coccyx (tailbone). In children, it consists of usually five unfused vertebrae which begin to fuse between ages 16-18 and are usually completely fused into a single bone by age 26.
It is curved upon itself and placed obliquely (that is, tilted forward).The base projects forward as the sacral promontory internally, and articulates with the last lumbar vertebra to form the prominent sacrovertebral angle. In Slavic languages and in German this bone is called the "cross bone".
Parts
The pelvic surface of the sacrum is concave from above downward, and slightly so from side to side.
The dorsal surface of the sacrum is convex and narrower than the pelvic.
The lateral surface of the sacrum is broad above, but narrowed into a thin edge below.
The base of the sacrum, which is broad and expanded, is directed upward and forward.
The apex (apex oss.sacri) is directed downward, and presents an oval facet for articulation with the coccyx.
The vertebral canal (canalis sacralis; sacral canal) runs throughout the greater part of the bone; above, it is triangular in form; below, its posterior wall is incomplete, from the non-development of the laminae and spinous processes. It lodges the sacral nerves, and its walls are perforated by the anterior and posterior sacral foramina through which these nerves pass out.

Articulations
The sacrum articulates with four bones:
the last lumbar vertebra above
the coccyx (tailbone) below
the ileum portion of the hip bone on either side
Rotation of the sacrum forward a few degrees vis-à-vis the ilia is sometimes called "nutation", and the reverse motion "counter-nutation."
It is called the sacrum when referred to all of the parts combined, but sacral vertebrae when referred individually.
Sexual dimorphism
The sacrum is noticeably sexually dimorphic (differently-shaped in males and females).
In the female the sacrum is shorter and wider than in the male; the lower half forms a greater angle with the upper; the upper half is nearly straight, the lower half presenting the greatest amount of curvature.The bone is also directed more obliquely backward; this increases the size of the pelvic cavity and renders the sacrovertebral angle more prominent.
In the male the curvature is more evenly distributed over the whole length of the bone, and is altogether greater than in the female.
Variations
The sacrum, in some cases, consists of six pieces ; occasionally the number is reduced to four .