Abstract: A corner lock for a mitered corner of a hollow door or window frame includes a pair of braces positioned in the interior of the door or window frame at each corner. The braces are interlocked with one another and extend from an outward part to an inward part of each corner. The first brace is directly attached to interior walls of outward horizontal and vertical parts of a door or window frame. The second brace has an outward end attached to the first brace and an inward end that abuttingly engages the inward corner of the door or window frame. A central screw extends through the first brace and bears against the second brace when advanced. The first brace bends into a convexity and the second brace bends into a concavity when the central screw is advanced. A rigid leg that forms a part of the second brace includes a base that bears against an inward corner when the central screw is advanced so that the mitered corner is strenghthened and locked against movement.