String line, stakes and the 3-4-5 knot

Right Angles and Setting Out on Site

A building's corner has to be 90°; a couple of degrees off at the corner grows into centimetres down the wall. But nothing on a site reads an angle — nobody carries a twenty-metre set square. You have stakes, string and a tape, and all three measure the same single thing: length. So where does the angle come from?