A scribing tool for marking wall panels for cutting them to fit the contour of a wall corner. The tool body has either a flat face or a rotating roller for engaging the corner wall surface that is perpendicular to the wall being paneled. The tool body carries a scribing element or pencil. In one species the scribing element is axially adjustable in a selected one of intersecting V-grooves in the top surface of the tool body, and a flat face is perpendicularly movable relatively to the tool body by being on the vertical face of an L-shaped strap whose medially slotted lower reach underlies the tool body. In a second species the scribing tool is axially adjustable in a V-groove in the upper surface of a disc rotatably mounted on the top surface of the tool body. In both flat-faced species, a rod passing vertically through the tool body clamps the scribing element and the adjustable body part in their adjusted positions by means of a nut on its threaded lower end. In a third species a roller is mounted on an axle pin fixed to a handle at an angle of 45 degrees. The axle pin terminates in a conical scriber point and is spring-retractable into the roller across the plane of its wall-engaging flat face. When the handle is held horizontally its center line passes close to the intersection line of the corner wall surfaces. A pair of sheet-metal gages having panel-gripping pockets of a depth equal to the amount of a preselected panel overlap, and also equal to the scriber-tool setting, are desirably employed.