Abstract: A pair of stop blocks have clamping elements for clamping the blocks in selected positions along a guide rail for a mat cutter and in predetermined relation to the edges of a mat to be cut. The stop blocks serve to precisely determine the start and finish of a line of cut. One of the stop blocks has a feeler rod adjustable thereon in a direction parallel to the guide rail for engagement with an edge of the mat board to thereby position the stop block to engage the cutter when the latter is a predetermined distance inwardly from the edge of the mat board.