Q: We are often advised by the experts to measure diagonally to determine if a project is square. Why is this? Wouldn’t it be easier and more accurate to put a square on the corner?
A: In small boxes and very simple assemblies, using a square would probably be sufficient. When projects get larger and more complex (for example, a cabinet that has several internal compartments), checking a couple of corners with a square does not give you a reading of the overall project — just those specific sections of the piece.
The corner-to-corner diagonal measurement technique is not only fast and extremely accurate, but it is also very helpful during the assembly process. As many of us have discovered, when you are in the middle of gluing up a project — a step that, as Ian Kirby likes to point out, is not easily reversible — it is not uncommon that the clamping pressure is applied unevenly and can actually distort the project out of proper alignment. One big advantage of measuring from corner to corner across a clamped-up subassembly is that you can quickly check to see if that is the case. If your project is a bit out of whack, you can change your clamp positions and clamping pressure while the glue is still wet and get the assembly square and true before the glue cures.