Accuracy in the workshop starts long before the first cut. If your square is not truly square, every joint inherits that error, and the mistakes compound as a project grows. The good news is that you can check and trust an inexpensive square without buying a precision reference tool.
The two-line test
To find out whether a square is genuinely ninety degrees, you only need a flat board with one straight edge.
- Hold the square against the straight edge and draw a fine line along the blade.
- Flip the square over so it faces the other way, line it up at the same spot, and draw a second line.
- If the two lines sit exactly on top of each other, your square is true. If they fan apart, the gap between them is double the actual error.
This works because flipping the tool doubles any inaccuracy, making a tiny problem easy to see. It is the fastest honest test there is, and it costs nothing.
Marking so the line means something
Even a perfect square is useless if your marking is sloppy. A thick pencil line can hide a sixteenth of an inch of ambiguity. Switch to a marking knife for joinery: it severs the wood fibers, gives you a crisp wall to register a chisel or saw against, and leaves no doubt about where the cut belongs.
Reference one face and one edge
Choose a single flat face and a single straight edge on each board, mark them clearly, and take every measurement from those surfaces. When all your squaring and marking comes from the same two references, small irregularities elsewhere in the board stop mattering. Parts that were measured from a common reference fit together cleanly even when the stock is not perfect.
Precision is a habit more than a tool. Verify your square, mark with a knife, and reference consistently, and a modest combination square will give you joints as tight as any expensive one.