Rule of Thumb

By Maeve Maddox

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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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10 Responses to “Rule of Thumb”

  • Cine Cynic

    Thank you, Maeve. Until now I was one of those who thought that the expression came from the “ancient law”. It is a very believable story.

  • Kevin

    For some reason I prefer the explanation in Bookdock Saint’s 😉

  • Zoran

    Thanks for the explanation. And yes, Wikipedia has all the info you desire for the term “rule of thumb”.

    Regards,

    Zoran

  • John | We Have Contact

    Hmmmm, I’m guilty of propogating the “ancient law” version. Guess I should stop. 😉

  • Maeve

    Cine Cynic is right – it makes a great story. I was rather disappointed myself to learn it was a myth.

  • write a writing

    I really got to subscribe to your blog because of these interesting back drop scenes that you reveal about the common stuff….

    Neat Work
    Cheers!

  • Dee

    I really don’t like that myth. It makes domestic abuse into a joke, and implies that it’s a primitive and natural human action.

  • K.M. Weiland

    Thanks for the link to article on body-parts measurement. Very helpful! I write historicals, so the terms for primitive units of measurement is important (but sometimes not so easy to find material on).

  • Africa

    The story seems much believable but i still have some doubt whether its really explains it all!
    I’ll continue to look for more information about it.

    However, this is a good job.

    Africa.

  • Africa

    The story seems much believable but i still have some doubt whether it really explains it all!
    I’ll continue to look for more information about it.

    However, this is a good job.

    Africa.

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