These two ligaments transform the greater and lesser sciatic notches into the greater and lesser foramina.

http://en.wikipedia.org/wiki/Pelvis

Additionally, these two ligaments transform the greater and lesser sciatic notches into the greater and lesser foramina, a pair of important pelvic openings.The iliolumbar ligament is a strong ligament which connects the tip of the transverse process of the fifth lumbar vertebra to the posterior part of the inner lip of the iliac crest.