Openings are rarely square, so measure the width at the top, middle and bottom, the height on the left, middle and right, and the two diagonals corner to corner. If the diagonals match, the opening is square; if not, we build for it. Use a steel tape and a helper. Photograph the opening and tell us what the top is (beam, header or ceiling), what it is made of, what is in the way, and what is at the bottom: tile, deck boards, a rail or a sill.