Carpenters and builders have used a Pythagorean trick for thousands of years to make perfect right angles without a square — just rope and a tape measure.
The 3-4-5 trick
Mark off 3 units along one wall, 4 units along the other. If the diagonal between those marks is exactly 5 units, the corner is a perfect 90°. If it's not, the wall isn't square — adjust.
It works because 3² + 4² = 9 + 16 = 25 = 5². Any multiple — 6-8-10, 9-12-15, 30-40-50 — does the same job, so you can use whatever scale fits the room.