The hip bone is formally called the os coxae (pronounced “oss COX-ee”), a Latin term meaning “bone of the hip.” You may also see it called the innominate bone, an older name that literally translates to “unnamed bone.” In everyday medical conversations, most providers simply say “hip bone,” but all three terms refer to the same structure: a large, irregularly shaped bone that forms each side of your pelvis.
Three Bones That Become One
Each hip bone is actually made of three separate bones that fuse together as you grow: the ilium, the ischium, and the pubis. In children and teenagers, these three bones are separated by Y-shaped cartilage inside the hip socket. They gradually merge during puberty, and the fusion is usually complete by about age 20. Once fused, they function as a single, solid bone.
All three bones meet at the acetabulum, the deep, cup-shaped socket on the outer surface of the hip bone. This socket is where the rounded head of your thigh bone (femur) sits, forming the ball-and-socket hip joint. That connection is what gives the hip its wide range of motion, letting you walk, run, squat, and rotate your leg in multiple directions.
The Ilium: The Upper Wing
The ilium is the largest of the three bones and makes up the broad, fan-shaped upper portion of the hip bone. Its curved upper edge, called the iliac crest, is the ridge you feel when you rest your hands on your hips. Because the iliac crest sits so close to the skin’s surface, it’s one of the most recognizable landmarks on the skeleton. Doctors use it to locate specific vertebrae for spinal procedures, and surgeons harvest bone graft material from it because the ilium contains a large amount of bone tissue and red marrow.
The ilium also connects to the sacrum (the triangular bone at the base of your spine) through the sacroiliac joint. This joint links the pelvis to the lower spine, transferring the weight of your upper body down through your hips and into your legs.
The Ischium: Your Sitting Bone
The ischium forms the lower back portion of the hip bone. Its most notable feature is the ischial tuberosity, the thick, rounded knob of bone at the bottom of each side of your pelvis. These are the bones you actually sit on, which is why they’re commonly called “sitting bones.” The ischium bears much of your body weight when you’re seated and serves as an anchor point for the large muscles of the thigh and hamstrings.
The Pubis: The Front Connection
The pubis is the smallest of the three bones and sits at the front of the hip bone. The left and right pubic bones meet at the midline of your body at a joint called the pubic symphysis, a firm but slightly flexible connection held together by cartilage. This joint allows a small amount of movement, which becomes especially important during pregnancy and childbirth. The angle formed where the two pubic bones converge (the pubic arch) is one of the key features doctors use to distinguish male from female skeletal anatomy.
Male and Female Differences
The hip bones look noticeably different depending on biological sex. A female pelvis is broader and shallower, with a larger opening to accommodate childbirth. The sitting bones are farther apart, making the pelvic outlet wider, and the pubic arch is more open. The hip sockets also sit farther apart and angle slightly inward. A male pelvis, by contrast, is narrower and deeper, with a more acute pubic arch. These differences are reliable enough that forensic scientists can identify biological sex from hip bones alone with a high degree of accuracy.
The Hip Socket and Thigh Bone
When people talk about “the hip,” they often mean two different things. The hip bone (os coxae) is the pelvic bone described above. But the hip joint is where that bone meets the femur. The ball-shaped head of the femur fits snugly into the acetabulum, and together they form what’s medically called the acetabulofemoral joint. A ring of cartilage lines the socket, and a capsule of ligaments holds everything in place.
This distinction matters in a practical way: what most people call a “hip fracture” doesn’t actually involve the hip bone at all. The vast majority of hip fractures occur in the upper part of the femur, either at the femoral neck (the narrow section just below the ball) or in the intertrochanteric region (a section marked by two bony bumps where major muscles attach). True fractures of the os coxae itself, such as a pelvic fracture through the acetabulum, are less common and typically result from high-energy injuries like car accidents.
Landmarks You Can Feel
Several parts of the hip bone are easy to locate on your own body. The iliac crest is the most obvious: run your fingers along the top curve of your pelvis from front to back. The bony point at the front end of that curve is the anterior superior iliac spine, a landmark tailors use to measure inseam length and physical therapists use to assess posture. If you sit on a hard surface and shift your weight side to side, the firm pressure points are your ischial tuberosities. And pressing into the front of your lower abdomen just above the groin, you can sometimes feel the firm edge of the pubic bone.
These landmarks help healthcare providers assess alignment, measure leg length, identify sources of pain, and plan surgical approaches. They’re also reference points for fitting prosthetics, orthotics, and even properly adjusting a bicycle seat to reduce pressure on the sitting bones.