The SI or Sacroiliac joint is between the tailbone (sacrum) and the hip bone (ilium). Unlike other joints, it doesn't move around allot. Instead, it's a viscoelastic joint that gets it's movement by giving or stretching. The amount of movement in this joint has been controversial, with some studies saying it moves allot, while others saying it doesn't move that much. Either way, since it's most important function seems to be acting as a shock absorber for the spine by stretching in different directions, the actual amount of movement is probably not that important.
This 1804 illustration shows the location of the SI Joint.
This
more recent image shows how the forces from the legs and the spine meet at the SI joints.
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