You want to square your project. Its to big or to small for a square. Maybe you can’t reach one corner to do a corner to corner comparison. Use the 3-4-5 rule.
The rule state simply if one side of a triangle is 3 and the other side is 4, the diagonal measurement will be 5. This can be inches, feet, millimeters, miles, arms lengths, anything as long as they are all the same.
Any set of numbers divisible by these will work. Need longer, double them for 6 – 8- 10.
Use the rule in conjunction to double check yourself.
Use it to check your square.
Use it to make your new “antique” wooden square.
-- Master hand plane hoarder. - http://timetestedtools.com