The Martin Prodraft Parallel Straightedge Kit is the premier item in its class. The only Parallel Straightedge Board utilizing an Anti-warp Aluminum body straightedge for strength, and stability. Notice the deep ribbed strips so that the unit can be adjusted over the table edge to bring the artwork into the users lap. Each side of the unit has large knobs for adjusting the straight-edge to align with the artwork/drawing and stay parallel. Board size to 16X.jpg21 inch.