SESKStuff Everyone Should Know
SESK: Stuff Everyone Should Know · sesk.info/wood-check-square-diagonals

How can you check that a rectangular cabinet or frame is square?

Measure both diagonals; equal means square.

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  1. Measure its two widths and compare them
  2. Measure both diagonals and see if they match ✔
  3. Weigh each corner joint

Why: A true rectangle has equal diagonals. Measure corner to corner both ways. If they differ, the piece is racked, so ease the longer diagonal inward until the two numbers match.

Huh, didn't know that: Woodcraft suggests testing a square by striking a line along a straight edge, then flipping the square to see if the lines match.

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What is a tree's favorite school subject? Geometree.
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References

Answer: Measure both diagonals and see if they match. A true rectangle has equal diagonals. Measure corner to corner both ways. If they differ, the piece is racked, so ease the longer diagonal inward until the two numbers match.

  1. Top 10 Tips for Accurate Woodworking (Woodcraft) www.woodcraft.com/blogs/learning-center/top-10-tips-for-accurate-woodworking
    If both measurements are equal, the corners are at 90° and the assembly is square.
    Checked 2026-10-10.
  2. Top 10 Tips for Accurate Woodworking (Woodcraft) www.woodcraft.com/blogs/learning-center/top-10-tips-for-accurate-woodworking
    If they differ, the piece is racked, meaning it's slightly skewed
    Checked 2026-10-10.
  3. Top 10 Tips for Accurate Woodworking (Woodcraft) www.woodcraft.com/blogs/learning-center/top-10-tips-for-accurate-woodworking
    test it by holding your square along a known straight edge and striking a line. Then flip it to confirm accuracy.
    Checked 2026-10-10.

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