The sacroiliac joint connects the sacrum (triangular bone at the bottom of the spine) with the pelvis (iliac bone that is part of the hip joint) on each side of the lower spine. It transmits all the forces of the upper body to the pelvis and legs. There is not a lot of motion in the joint and it is very strong and stable.

Learn more at http://www.spine-health.com/conditions/spine-anatomy/sacroiliac-joint-anatomy