The =ilium= (Os ilium) is the largest of the three parts. It is
irregularly triangular and presents two surfaces, three borders, and
three angles.
The =gluteal surface= (Facies glutæa) faces upward, backward, and
outward. It is wide and concave in front, narrower and convex behind.
The wide part is crossed by the curved =gluteal line= (Linea glutæa),
which extends from the middle of the inner border toward the external
angle. This surface gives attachment to the middle and deep gluteal
muscles.
[Illustration: