Hi Matt, I bought a square from Home Depot a few years ago, which essentially looks like an L and is perfectly square at the 90 degree angle at the L.
It's made of metal, steel, I believe, and it's about five feet long. I find it great as a cutting guide, especially when I have to saw wood or drywall using a saw. Since the saw will go through the path of least resistance, I find it easy to follow the length of the square to cut any material I need to cut straight. Obviously, this would not do to cut metal, since both the material would be made of the same stuff, but it does make for a nice straight edge to cut other material with. Victor Gouveia
