The pronator quadratus is one of three muscles in the deep compartment of the anterior muscles of the forearm.
The pronator quadratus, as the name suggests is a square-shaped and flat muscle which is located in the distal forearm and assists in pronation of the forearm, pulling the distal end of the radius anteriorly over the ulna.
Anterior Interosseus Nerve (from median nerve)
Anterior interosseus artery (ulnar artery → common interosseus artery → ant. interosseus artery).
The pronator teres is sometimes absent but this is not functionally significant due to the other forearm pronator, the pronator teres.