Hi Peter,

Surely a square must be within a certain limit, albeit a fairly tight one. Otherwise you are suggesting that every atom is perfectly in line, and that they are all perfectly at 90.00000 recurring degrees relative to the other part. Not only would that be difficult to build, it would be extremely expensive. As soon as you picked it up you would have to reject it due to the heat from your hand distorting the frame if that was your pass or fail criteria.

We need to use limits all the time in Engineering - think of all those tolerances we put on dwgs. They aren't to make it perfectly square, they are to make it square enough for the intended application.