The 3-4-5 rule is a fundamental geometric principle derived from the Pythagorean theorem that builders use to ensure a deck's framing corners are perfectly square (at a ninety-degree angle). By measuring three feet down one side and four feet down the intersecting side, the diagonal distance between those two points must measure exactly five feet. If the diagonal measurement matches precisely, the corner is square; if not, adjustments are made before securing framing joists.
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