Abstract: A channel extending longitudinally through an extension bar snugly but slidably receives the base flange of a speed square. The extension bar can be locked in a desired position relative to the square, and has mechanism for mounting a stop peg at a desired position along the length of the bar. The position of the square relative to a board to be marked or cut is set by engaging the stop peg against the end of the board while the extension bar is held flush against a longitudinal edge of the board. Each of several different boards can be marked at a predetermined desired length and/or an edge of the speed square can be used as a guide fence for a portable power saw.