[Cerium]

And oh how we take it for granted. I recently spent a few minutes trying to make sense of a situation where I was using a corner of a paper for a square. It turned out the piece of paper was not at all square, at least a quarter of an inch out of square!

[bigiain]

One important lesson I remember from high school woodworking class ~45 years ago - when using a set square, make your markings twice with the square flipped over in the opposite direction, so if the square isn't accurate you'll get two distinct markings - and for most wood working purposes just splitting the difference by eye will be accurate enough.