Abstract: A setback gauge for the marking and installation of molding and trim around structural framed openings provides a method of accurately marking the inner corner points of such molding and trim without the need for measurement, interpretation of measuring or other scales or adjustment of any tool or device. The gauge further provides an accurate and uniform edge setback distance when installing such trim. One embodiment includes a securing arm which allows the gauge to be temporarily installed in the frame without marring the frame in any way and thus allows the marking and installation of trim to be carried out by a single person without need to hold the gauge in place during use. An alternative embodiment provides a simpler gauge of monolithic form which is used just as the gauge of the first embodiment, but does not provide any method for retention in the frame.