Rule of Thumb
Reader Cynthia Turney asks
Do you know where the phrase “rule of thumb” came from?
This is a figurative expression that means “a general guideline that has a broad application, but which is not strictly accurate in every case.”
I hope that by now everyone knows that this expression does not come from “an ancient law” limiting the circumference of the stick a man could use to beat his wife.
The expression originates from some forgotten literal context in which a craftsman or farmer used his thumb (about an inch) as a unit of measurement.
The word “rule” in this expression does not mean “principle” or “maxim” as in Robert’s Rules of Order. It has more to do with “ruler,” meaning something to measure with or “a strip used for making straight lines.”
Body parts have long been used as units of measurement. See the DWT post “Body Parts As Tools of Measurement.”
This Wikipedia article on “rule of thumb” will tell you all you can want to know about the expression’s false etymology. (There used to be an article on this expression at Snopes.com, but I couldn’t find it last time I looked.)
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