How do you check if a surface is square?

To determine if a surface is square, measuring diagonally from corner to corner is the most effective method. This technique involves measuring the distance between opposite corners of a rectangle or square. If both diagonal measurements are equal, this indicates that the corners are at right angles and the surface is square. This method is based on the principles of geometry, specifically the Pythagorean theorem, which asserts that the sum of the squares of the lengths of the two shorter sides of a right triangle must equal the square of the length of the hypotenuse.

The other options do not provide reliable methods for checking squareness. Measuring the thickness of the wood or comparing the widths of adjacent boards does not provide information about the angles between sides. Ensuring all sides are equal might apply to certain geometric shapes, like a square, but does not confirm right angles. Therefore, measuring diagonally from corner to corner remains the most dependable approach to verifying that a surface is square.