The 3-4-5 Rule

A long-held and very simple method for determining square is to use the 3-4-5 rule. This rule uses the Pythagorean Theorem (don't worry, you won't have to do any complicated math) to create a square corner. The principle is simple and very easy to implement, whether you're dealing with a very small project or building a house. Learn to use the 3-4-5 rule to square up your woodworking projects.

Source: http://woodworking.about.com/od/gettingstarted/qt/345Rule.htm

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