I didn’t have to drop the last framing square I bought to knock it out of square – it came from the store that way. So I chalked that up to Murphy’s Law, took it back and got another one from a different manufacturer. It too was out of square! While whining about this problem to a fellow woodworker, he looked at me like I was dumb and said, “You mean you don’t know how to true up your square?” Ten minutes later I was in my shop testing out his advice with a hammer.
First determine which direction the square is out of true – to the inside (89°) or outside (91°). Use a pointed steel punch (an old nail set also will work) and a good-sized hammer to strike the square at either the inside or outside corner of the square, and check it again for squareness. Striking to the inside corner will widen the square, while striking to the outside corner will constrict the square. If it isn’t fixed the first time, simply hit it again.
Now when I drop my framing square I don’t get nearly as frenzied. I just head for the punch and hammer.