On another woodworking forum, someone said that after building a case, you should measure the diagonals to ensure the case is square and that just checking if all the corners are 90 degrees won't suffice. Is it possible for a polygon to have four 90 degree corners and not be a rectangle? This makes no sense to me.
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$\begingroup$You are correct that a quadrilateral with four 90 degree corners MUST be a rectangle, in math. End of story.
In woodworking, however, one must deal with real measurement errors. It turns out that measuring the diagonals is a more sensitive method of measuring how close to 90 degrees one got, as opposed to measuring the angles with a protractor or other tool.
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