A guide preferably of small size preferably two and a half inches square is disclosed for establishing reveal on doors and window frames during molding installation without having to do measuring and marking. In one embodiment two square blocks making up the guide are fixedly attached to each other and a different guide is used for different molding reveals. In another preferred embodiment of the present invention, the position of the two blocks relative to each other is adjustable such that one block moves along the other block only along a diagonal providing a simple single adjustment which provides the same amount of reveal on two orthogonal sides of the guide.