The sacrospinous ligament is a thin, triangular ligament located deep to the sacrotuberous ligament with which it blends medially.  The fibers pass laterally from its broad attachment to the lateral margins of the lower sacral and upper coccygeal segments and converge to the apex to attach to the ischial spine.  Its anterior surface is muscular, constituting coccygeus, while the posterior surface is ligamentous.  The ligament forms the inferior margin of the greater sciatic foramen and the superior margin of the lesser sciatic foramen.