The thigh bone, femur, and the pelvis, acetabulum, join to form the hip joint. The hip joint is a "ball and socket" joint. The "ball" is the head of the femur, or thigh bone, and the "socket" is the cup shaped acetabulum.
The joint surface is covered by a smooth articular surface that allows pain free movement in the joint.
The cartilage cushions the joint and allows the bones to move on each other with smooth movements. This cartilage does not show up on X-ray, therefore you can see a "joint space" between the femoral head and acetabular socket.
The pelvis is a large, flattened, irregularly shaped bone, constricted in the center and expanded above and below. It consists of three parts: the ilium, ischium, and pubis.
The socket, acetabulum, is situated on the outer surface of the bone and joins to the head of the femur to form the hip joint.
The femur is the longest bone in the skeleton. It joins to the pelvis, acetabulum, to form the hip joint.