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Carpenters 345 Rule 
Carpenters use the 345 rule to insure layouts are square. It works like this. If the short side of the triangle is 3 feet, and the leg that extends from it 90 degrees is 4 feet, the hypotenuse, or longest leg, will be 5 feet.
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This basic principal of geometry has applications in many different carpentry and building tasks. Laying out concrete forms, setting posts for decks. Being sure a frame is really square. Units do not have to be in feet. Units may be in inches, meters, centimeters, millimeters, miles, kilometers or rods. It doesn't matter. If it measures 3 on one side and 4 on the second, it will be five feet on the long leg if the angle X is 90 degrees. This standard rule, for those interested, can be confirmed by the Pythagorean theorem which states that for a right triangle A^{2} + B^{2} = C^{2} where A and B are the length of the two sides that join at the right (90 degree) angle and C is the side that joins side A to side B. Therefore, for the 345 rule, A^{2} + B^{2} = C^{2} = 3^{2} + 4^{2} = 5^{2} = 9 + 16 = 25. This confirms the 345 rule because the length of side C will only be 5 if the angle where A and B join is exactly 90 degrees, or square.
