# How to lay out right angles in construction

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Laying out accurate right angles on building projects — such as foundations for sheds, decks or patios — is easy if you use geometry.
According to the Pythagorean Theorem, the square of the two sides of a triangle that adjoin the right angle (legs) are equal to the square of the third side (hypotenuse). This is expressed mathematically as a² + b² = c².
To use, multiply the length of each leg of the triangle by itself then add the two sums together to find the length of the hypotenuse when the angle is at 90°.
The easiest way to accomplish this is to use the 3-4-5 method:
Measure 3 feet out from the angle you want to make 90° in one direction.
Measure 4 feet out from the angle you want to make 90° in the other direction.
Measure across the two points and adjust the angle until the distance on the third side of the triangle is 5 feet.
You can also use multiples of 3-4-5 in the same ratio (such as 6, 8, 10) to form larger or smaller right angles.
Watch the video for find out more.