The pelvis is the bony structure located at the base of the spine (properly known as the caudal end). The pelvis incorporates the socket portion of the hip joint for each leg (in bipeds) or hind leg (in quadrupeds). It forms the lower limb (or hind-limb) girdle of the skeleton. The 'hip bone' is also known as the innominate bone.
The pelvis is symmetrical and each side is actually made up of three separate bones - the ilium, ischium, and pubis. These three bones fuse together with age and are collectively known as the ossa coxae. The pelvis is joined to the sacrum bone by ligaments, and the hip bones nest in specially shaped sockets (the acetabulum on each side. The place at the front of the pelvis where the two sides join together is called the symphysis pubis. This is normally a very inflexible joint, but it softens and becomes more flexible during late pregnancy, allowing it to expand during labour for the baby's head to pass through. A female pelvis is also wider and shallower than a male pelvis.
One easy way to distinguish between genders is to examine the very front of the pelvis, and compare the angle formed by the bones that come from below with your fingers. If the angle is about the same as between your outstretched thumb and index finger, it is a female pelvis (arcus pubis). If it is closer to the angle between your spread index and middle fingers, it is a male pelvis (arcus subpubis).
See also: renal pelvis, pelvic girdle