Square a Real Corner

Grab a tape measure and test any wall corner in your home. Measure 3 feet along one wall and mark the spot. Measure 4 feet along the adjoining wall and mark that spot. Now measure the diagonal between the two marks.

If the diagonal is exactly 5 feet, the corner is a true 90°. If it is 5 feet 1 inch or 4 feet 11 inches, the corner is off by several degrees. Professional framers use this "3-4-5" check before nailing anything because a one-inch error at the base becomes a foot of drift across a long wall.

The math is pure Pythagoras: 3² + 4² = 9 + 16 = 25 = 5². No calculator needed. Carpenters have relied on this field trick for generations — some historians think ancient Egyptian rope-stretchers may have used knotted cords the same way — and it still beats most digital tools for speed.