There are times when common knowledge can get you out of a jam. For example, did you know that Moss grows on the north side of trees?
The site Rules Of Thumb defines a rule of thumb as a homemade recipe for making a guess -- an easy-to-remember guide that falls somewhere between a mathematical formula and a shot in the dark. They help you appraise a problem or situation. They make it easier to consider the subtleties of the topic at hand; they give you a feel for a subject. And they make excellent cocktail party fodder.
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