Jevons Tool's 3D Squares ($60) are sold in sets of four to help with clamping casework corners and making joinery square, but they also have other uses. The machined aluminum legs are guaranteed to be within +/- .002 degrees of 90 degrees along their entire 6-inch length, meaning that they can be used to square up fences and blades, too. With a contact surface more than 1 inch wide, they balance better than most squares on machinery tables, so you have two hands free to make adjustments. A 2-inch leg version called the Mini 3D Square is making its debut soon. It's ideal for small jobs like squaring the base of a jigsaw to its blade or gluing up wooden toolbox drawers. 913-384-0023,