Original Article: http://www.mayoclinic.com/health/medical/IM00039
The sacroiliac joints link your pelvis and lower spine. They're made up of the sacrum — the bony structure above your tailbone and below your lower vertebrae — and the top part (ilium) of your pelvis. There are sacroiliac joints in both the right and left sides of your lower back. Strong ligaments hold these joints in place. The sacroiliac joints support the weight of your upper body when you stand.