How Screens Are Sized

A TV labeled "55-inch" isn't 55 inches wide or tall. That number is the diagonal, corner to corner, and it comes straight from Pythagoras.

Take a screen 48 inches wide and 27 inches tall. The diagonal is √(48² + 27²) = √(2304 + 729) = √3033 ≈ 55 inches. Every screen size in every store is measured this exact way, so two TVs with the same label can still have very different shapes.

Builders need square corners without carrying a protractor, so they use the 3-4-5 rule.

Measure 3 feet along one wall and 4 feet along the other. Then measure the diagonal between those two marks. If it's exactly 5 feet, the corner is a perfect 90°. If it's off, the corner is off. It works because 3² + 4² = 5², and you can check a foundation, a deck, or a picture frame with nothing but a tape measure.