3-4-5 rule for squaring
Vince Stanley writes:
For squaring up a deck, foundation, or anything else that needs to be square, use the 3-4-5 rule.
The best place to start is in one corner of the layout. The corner should be a right angle with two legs. The end of the left leg is A, the corner is B, and the end of the right leg is C. Tie a string between A and B to create left leg A, and tie a string between B and C to create right leg C. B is the corner.
To apply the rule, start at stake B and measure out 3 feet toward A. Mark the string. Beginning again at stake B, measure out 4 feet toward C. Mark the string. With a second pair of hands and starting at your 3-foot mark on string A, pull a tape measure to the 4-foot mark on string C. This diagonal measurement should read exactly 5 feet.
When working alone, put a stake on the outside of the string at the 3-foot mark on A and tie a string to the stake. Mark this string with a 5-foot measurement. Now, pull the string so that the 5-foot mark lands on your 4-foot mark on C. If the distance between A and C doesn't measure 5 feet, then move A or B out or in until it does. When the measurement between the two points (A & C) reads 5 feet, your layout is square! For larger projects simply use a larger number combination making all three numbers divisible by 3-4-5, i.e., 21-28-35 (21÷3=7, 28÷4=7, and 35÷5=7).