The sacroiliac joints reside in between the sacrum (the tailbone section with the spinal column) as well as the popular wing-like iliac bones that variety the pelvic girdle. There are 2 SI joints, just one on the left and 1 on the ideal (highlighted in pink in the image over), https://www.theorthopaedicandpainpractice.com/service/hip-groin-pain/