The pelvis is a balanced and structured part of the body.  Fused by three  bones: the ischium, the ilium, and the pubis it is fairly symmetrical.  The sacro-iliac joint is formed in the area where the tow bones the sacrum and Ilium meet.  The area is a synovial joint.  A synovial joint is the type of joint found between bones that move against each other, such as the joints of the limbs (e.g. shoulder, hip, elbow and knee). Characteristically it is a cavity filled with fluid. Together with the ligaments, tendons and muscles in the area, the capsule keeps the bones of the joint in place.  Although this joint does not have a huge amount of movement its got limited mobility.

Sacrum means sacred from the Latin.  Yogis believe this is where our untapped potential lies dormant. It is where the seat of kunadili where we engage in yoga to access the areas energy to harness and resource the creative energy to create our world around us.  When we stand up it locks in position as it is designed to give a firm base of support to the spine and trunk.  When we sit it unlocks and can be come unstable and irritated.  When we are young its smooth but as we age it becomes bones and suffers from more interlocking challenges.  It moves in a gliding, rotation and tilting movement.