13/16" for the skirts of my shop (one for wood and one for welding/metalworking). Both squares came with notches along the inside edge of the blades, just enough to allow several thousandths of an inch of daylight when I insert one square into the other. I MAY be able to place the squares under magnification and remove the nicks with a fine water stone, but 1) I don't have to, and 2) I'm not sure I should even try. Even with the barbs the little 2" bugs are GREAT for a quick check on woodworking, welding, assembly etc but I wouldn't even try to certify them for a tool shop. 4 star product because of nicks otherwise 5 stars.
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