The sacroiliac joints reside between the sacrum (the tailbone phase from the spinal column) as well as the prominent wing-like iliac bones that sort the pelvic girdle. There are 2 SI joints, just one on the still left and 1 on the right (highlighted in purple in the impression above), https://www.theorthopaedicandpainpractice.com/service/hip-groin-pain/