The lower circumference of the lesser pelvis is very irregular; the space enclosed by it is named the inferior aperture or pelvic outlet. It is an important component of pelvimetry.

It has the following boundaries:

• anteriorly: the pubic arch
• laterally: the ischial tuberosities
• posterolaterally: the inferior margin of the sacrotuberous ligament
• posteriorly: the anterior border of the middle of the coccyx.^[1]

These eminences are separated by three notches:

• 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.

• Pelvic inlet

• 
  Articulations of pelvis. Anterior view.
• 
  Articulations of pelvis. Posterior view.
• 
  Low-dose CT scan of sagittal pelvic outlet diameter as part of pelvimetry
• 
  Intertuberous diameter, as a measure of the transverse measure of the pelvic outlet

• pelvis at The Anatomy Lesson by Wesley Norman (Georgetown University) (medialpelvissmall, malepelvissagittalinlet&outletsml)