The pelvis is divided into the false or gretaer pelvis and the true or lesser pelvis by an oblique plane that extends from the upper anterior margin of the sacrum;  along the arcuate line to the upper margin of the pubic symphisis. The boundary of this line is called the pelvic brim, which delineates the boundaries of the abdominal and pelvic cavities. The region above the brim is called the false pelvbis and the region below the brim is called the true pelvis, which can be subdivided  by the pelvic diaphragm, into the main pelvic cavitiy and the perineum.