The sacroiliac joints reside in between the sacrum (the tailbone phase with the spinal column) plus the prominent wing-like iliac bones that sort the pelvic girdle. There's two SI joints, 1 to the left and just one on the right (highlighted in crimson during the graphic over), and combined with https://www.theorthopaedicandpainpractice.com/service/hip-groin-pain/