Abstract: A mortise guide and cutter for mortising a workpiece is provided. The mortise guide comprises a frame which has a rectangular aperture, means for positioning the frame to engage an edge of the workpiece, and means for attaching the frame to the workpiece. A sizing blade, which is pivotally mounted to the frame, is configured to be selectively rotated from a first position exposing the entire aperture to a second position reducing one dimension of the aperture by the width of the blade.