Acetabulum articulates 

The acetabulum is a concave socket located on the lateral surface of the hip bone, which is one of the bones that make up the pelvic girdle. Hip bone (Innominate bone) is formed by the fusion of three bones: the ilium, ischium, and pubis.

The head of the femur fits into the acetabulum, creating a ball-and-socket joint. This type of joint allows for a wide range of motion in multiple directions, making the hip joint one of the most flexible and essential joints in the human body.