I got this because I didn't have what I would call a true 45 square. I was setting up my wood working saws and what not using a plastic or aluminum rafter square and wondering why things didn't always come out like they should, after all it's wood on not metal, right? After getting my lathe and mill I realized that what I was using wasn't probably as true as it could be. I did square up the squares that I had been using with my mill. So now all is good and I have a, as true as it gets, 45 to check and use to set up with.