The sacroiliac joints (SI joints) are formed by the sacrum and pelvis. The sacrum is like the tailbone at the base of the spine fits in between the two pelvis bones which are called the iliac bones. This joint is not a highly mobile joint because its role is largely one of providing stability and it is weight bearing in nature. Therefore it is held together by strong ligaments. Because this joint in under a lot of stress it is a common area to experience discomfort in and this is termed SI joint pain.