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There are two sacroiliac (SI) joints. They are in the very low back (buttocks area). There is one joint on either side of the pelvis. The SI joints link the sacrum to the ilium. The sacrum is a triangular bone at the bottom of the spine. The ilium is part of the pelvis. The SI joints are held in place by ligaments. Ligaments are strong tissue that link bone to bone. The joints do not move very much. But they do help with mobility. They help you flex, bend to the side, and twist. They also help bear the weight of your upper body.