What is the best way to check a square or rectangle for squareness?

• A. Use a level to verify squareness.
• B. Measure diagonals.
• C. Check corners with a carpenter's square.
• D. Measure the sides and compare lengths.

Answer: (B)

Measuring diagonals is the quickest and most reliable way to verify squareness of a square or rectangle. In a true square or rectangle, the two diagonals run from opposite corners and should be the same length. If you measure both diagonals and they match within tolerance, you’ve confirmed that the corners come out right angles and the shape is square or rectangular. If the diagonals differ, something’s off with the angles, indicating the figure isn’t square. This method is often faster and more comprehensive than just checking level or trying to infer squareness from side lengths, which can miss small misalignments spread across the frame.