If you want one that is very accurate, get a Moore and Wright square. These will be more than sufficient for engineering purposes.
check the Conformance Certificate.
I would avoid buying a second hand square as if they are dropped it can knock the accuracy.
Thank you SS. I have been looking at M&W as I know they are a well established firm, I knew of them when I was doing my apprenticeship at Linwood. How do I see the Conformance Certificate?
I certainly wouldn't buy second hand,