The acetabulum is a deep, cup-shaped, hemispherical depression, found in our hip. It is formed by three bones, the pubis, the ischium; and the ilium. (Details from Gray's Anatomy, found at Wikipedia)
Little O had an excellent idea. He wants to make an A, B, C book about our bones. We are going to try to research and post one letter a day. We will be relying heavily on Gray's Anatomy on the web. Wish us luck!