Hi Irving,

I guess there are several areas of a square which need to be accurate. The internal angle, the external angle, the straightness of the inner edge on the blade and the inner edge on the stock and the straightness of the outer edge on the blade and the outer edge of the stock.

When you compare a square with the 321 block you are using the internal angle, which sounds like it might be 90deg on all the squares in the 'corner'. When you compare one square to another you must be using the outer angle on one vs the inner on the other. If they are different then the outer edge is not parallel to the inner edge.

All this suggests that you can't rely on any of them for the outer angle, only perhaps the inner. Were they expensive?

BS939 covers the specification for squares, but at £141 :surprised: I don't own a copy ! But I do know that the square marked 'conforms to BS939' has given good service over the years and seems to be OK.