The 3 4 5 method is an old-school but clever way to make sure corners in construction are a true 90 degrees. It's all about using simple math—think triangles and a tape measure—to get your building layout straight from the start. This method pops up everywhere from huge commercial projects to backyard sheds. You'll learn why it works, how to use it even if you've never picked up a protractor, and what can go wrong if you get sloppy. Read on to avoid common layout headaches and get crisp, square corners every time.