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The sacrum articulates with the ilium.  

Explanation:

The sacrum is a bone formed by the sacral vertebrae, which have fused to give this bone. Above it articulates with the lumbar vertebra, below with the coccyx, which is the last section of the vertebrae column. Lastly, it articulates with the ilium at the sides. The ilium is one of the three bones that makes the hip bone. It is the largest one and in the upper part of the hip bone.

The ischium and pubis, which are the other two bones that form the hip bone, do not articulate with the sacrum, but they articulate with the ilium and with each other.

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