one in front, the pubic arch, formed by the convergence of the inferior rami of the ischium and pubis on either side.
The other notches, one on either side, are formed by the sacrum and coccyx behind, the ischium in front, and the ilium above; they are called the sciatic notches; in the natural state they are converted into foramina by the sacrotuberous and sacrospinous ligaments.
When the ligaments are in situ, the inferior aperture of the pelvis is lozenge-shaped, bounded as follows:
in front, by the pubic arcuate ligament and the inferior rami of the pubes and ischia
laterally, by the ischial tuberosities
behind, by the sacrotuberous ligaments and the tip of the coccyx.