I hadn't had much luck with verifying the 'squareness' of my squares. As it's turned-out of my assortment of 10 squares ... I have 2 12" adjustables are dead-on ... as I can measure. The rest are all 'out' in varying amounts. I've been stumped about how to measure the 'squareness'. While reading thru the "Which Parallel Guides for Makita Track Saw" thread: something caught my-eye in one of the 'attachments' and decided to try a set-up comparing the square to itself.
DSC_1006.jpg
This is a variant to the 'flip' method. The picture shows the square in the flipped position with a .036 feeler guage ... showing that the square is .018 out-of-square. Using a Veritas straight-edge for a base ... the square was first placed at the opposite side of the pins (2, 1/2" shank router bits) and the pins aligned to touch the square. When flipped: the square was slid to touch the 'nearest' pin ... the gap to the 2nd pin being the 'doubled' amount of out-of-square.
I'm looking forward to seeing posts of other methods.