The upper part (short posterior sacroiliac ligament) is nearly horizontal in direction, and pass from the first and second transverse tubercles on the back of the sacrum to the tuberosity of the ilium. The lower part (long posterior sacroiliac ligament) is oblique in direction; it is attached by one extremity to the third transverse tubercle of the back of the sacrum, and by the other to the posterior superior spine of the ilium.