The 3-4-5 rule, scaled to any size
A corner is square when its diagonal equals <strong>√(A² + B²)</strong> — the Pythagorean theorem applied to decking, foundations, fences and tiling. The classic 3-4-5 triangle scales up: 6-8 gives a 10-foot diagonal, 30-40 a 50-footer. Foundation crews stretch strings and check both diagonals of the rectangle.
Check both diagonals
One matching diagonal only proves the shape is a parallelogram — a rhombus has equal sides and unequal angles. <strong>Measure both diagonals</strong>: when they're equal to each other (and to the calculated length), the rectangle is truly square. Tape measure, two people, five minutes — this is the highest-leverage check in construction.