A straightedge aids ripping or routing a large sheet of plywood but it takes time to adjust the straightedge so the saw blade or router bit cuts exactly on your layout line.
shop tip

Make a smarter straightedge by gluing a 2" wide strip of 12 "-thick stock to an equally long piece of 14 " plywood or tempered hardboard. Both pieces should be a few inches longer than your longest cuts, and the 14 " piece should be wider than the distance from your saw's blade (or router's straight bit) to the edge of the tool's base. When the glue dries, rip or rout the 14 " stock as shown above. Now, you can align this just-cut edge with your layout mark to know where the cut will start.
—Roger Boulet, Winnipeg, Manitoba