One way This Old House general contractor Tom Silva ensures that mason's strings and deck framing are square when he is building a deck is by employing the 3-4-5 rule (based on the Pythagorean theorem): First, he tacks a double joist hanger to one end of the ledger and slips a side rim joist into place. Then he marks the top of the ledger 3 feet from the corner and the top of the side rim joist 4 feet out. Finally, he extends his tape measure between the two marks and moves the rim from side to side until the marks are exactly 5 feet apart. At this point, the two pieces are square. Larger triangles, with sides of 6-8-10 and 9-12-15, yield even more accurate results.