The ilium is a paired bone of the pelvic skeleton, and the uppermost and largest part of the hip bone.

Each ilium has two parts:

• Body of the ilium - the central portion situated near the acetabulum. It contributes to forming around 1/3 of the acetabulum, also forming part of the acetabular fossa. The internal surface of the body of the ilium forms part of the lesser pelvis and serves as the origin site of some fibers of the obturator internus muscle.
• Wing of the ilium - the large expanded portion of the ilium. The wings of the ilium bound the greater pelvis laterally.

The ilium also presents several important landmarks, such as the:

• Arcuate line - a prominent bony ridge on the internal surface of the ilium separating the body from the wing, also forming the boundary between the false pelvis and true pelvis;
• Iliac crest - the superior border of the iliac wing, which features four bony projections:
  • Anterior superior iliac spine
  • Anterior inferior iliac spine
  • Posterior superior iliac spine
  • Posterior inferior iliac spine
• Iliac fossa  - a concavity forming the internal surface of the wing of the ilium;
• Sacropelvic surface, featuring:
  • Auricular surface - an ear-shaped surface within the sacropelvic surface of the ilium, which articulates with the sacrum;
  • Iliac tuberosity  - a roughened area behind and above the auricular surface;
• Gluteal surface, featuring:
  • Anterior gluteal line
  • Posterior gluteal line
  • Inferior gluteal line