A crate corner looks wrong, so you reach for the square. But a dropped tool or a loose handle can leave the square itself out of true. Before trimming a slat to suit a doubtful reading, give the tool a quick check on scrap. It is a smaller job than replacing wood you did not need to cut.
Choose a flat scrap panel with one known-straight edge. Brush away dust and check that neither the edge nor the stock of the square has a dent where they meet. This test depends on a reliable reference edge; a rough offcut straight from the scrap bin may only add another source of error.
Seat the stock of the try square firmly against that edge and draw a thin pencil line along the blade. Flip the square over so the stock still registers against the same edge, then bring the same blade edge back to the starting point. Draw a second line beside the first with a tiny gap so you can compare them clearly.
The two lines should stay parallel. If the gap opens toward the far end, repeat the test with the stock fully seated and the pencil held consistently. A thick pencil mark can hide a small difference. If the divergence repeats, set the tool aside for adjustment according to its maker or replace it; do not bend the blade by guesswork.
With the square checked, return to the crate. Register it against a clean, straight face rather than a rounded edge or a proud screw head. On a rectangular frame, comparing both diagonals is another useful check, but neither a neat corner nor matching diagonals proves that a repaired crate is safe to carry a load.
Practical takeaway: draw the flip test on a spare panel before an important layout job or after dropping a square. Keep the panel as a quick reference, and check the measuring tool before removing wood to match it.
Hand ToolsLayoutCrate Repairs