The sacrum is a large triangular bone towards the lower end of the spine. This is formed from 5 fused sacral vertebrae. The widest, upper area of the sacrum, articulates with the pelvic girdle through the sacroiliac joints on each side. The narrower base of the sacrum articulates with the coccyx and together they form the back of the pelvis.
The sacrum spreads the weight of the torso across the arch formed by the pelvis, before it is carried by the legs and feet when we stand and by the sitting bones whilst we sit.