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Where does the phrase rule of thumb originate from? Why thumb of all possible body parts ;-)

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Wikipedia answer: en.wikipedia.org/wiki/Rule_of_thumb –  Gilead May 11 '11 at 14:56
    
argh, wikipedia, didn't think would find it there, thank you. –  n002213f May 11 '11 at 15:11
    
It's nothing to do with beer either. –  user362 May 11 '11 at 17:57

5 Answers 5

No one knows. The expression has existed in many languages for a long time, which suggests that its origin is pretty old.

There are several theories, some based in the similarities in many languages between the words inch and thumb and how you can measure an inch using the thumb, others based on the general usefulness of the thumb to measure different things.

It's entirely possible that it originally had nothing at all to do with the thumb; that it was a similar word that has become distorted over time, then translated to other languages in its distorted form.

See also: Rule of thumb on Wikipedia.

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Interestingly, in Italian you would say a spanne to indicate something approximate, where spanna means span (in the sense of the measuring unit en.wikipedia.org/wiki/Span_%28unit%29 ). This may support the "measuring theory". –  nico May 11 '11 at 17:59
    
And the "wife beating" folk etymology and many others. Like "the whole 9 yards" and "down to brass tacks," this one is widely disputed and there is no accepted "official" answer to it. Dollars to donuts it has nothing to do with the human thumb. –  The Raven May 11 '11 at 19:49

Perhaps it's because the length of the thumb joint to the end of the thumb is a fairly accurate representation of an inch. So rule of thumb was likely a way to quickly verify the measurement before cutting for construction work rather than search for a yard stick. The meaning probably began to be used in a more abstract sense as a rule to quickly validate something.

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Good, glad you found the answer to your own question. –  Neil May 11 '11 at 16:01
    
If there is one thing that can be said about the etymology of this expression, it's that the above is not it. –  The Raven May 11 '11 at 19:51
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And you've determined this based on the information written on a single internet page? I hope I don't offend you too much when I say I don't put much weight in your judgement. –  Neil May 15 '11 at 20:29
    
This is the etymology that's most commonly prescribed for the phrase, that your thumb makes a rough "ruler", so, the "rule of your thumb" was about an inch. –  user20276 Apr 18 '12 at 21:13

If you ever do any carpentry you can get pretty close with your thumb as a rule / gauge.

So any measurement not using actual instruments is by "rule of thumb", or "by eye".

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Not sure if it helps, but this expression reminds me of basic Physics lessons in secondary school: Fleming's "rule of thumb"/right hand rule (similar Oersted's rule in the wiki article posted by Guffa)

Prediction of direction of field (B), given that the current I flows in the direction of the thumb

Prediction of direction of field (B), given that the current I flows in the direction of the thumb

(via wikipedia)

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Rule of Thumb is an undocumented, implied rule to abide.

One possible origin, or at least implementation, is found in the 117th New Constitution of Roman Emperor Justinian I, published in 529 C.E., granting a husband freedom to "beat his wife with a whip or rod" for divorcable offenses. The "rule of thumb" was possibly conjured to suppress nefarious abuse of the law. This rule has never been documented, as it is merely a guideline, to help prevent serious injury.

An example rule of thumb is: "Slice a peach along its suture, to remove the pit." There's no harm in bisecting the fruit along any other geodesic. It's simply easiest to remove the pit when bisected in a specific manner. It's also a good rule to not call the suture of the peach its "butt crack" (or worse), as this will likely result in a negative impact to social status among your peers.

The failures of man to abide by rules of thumb resulted in the invention of posting warning signs for the most unlikely and oddly specific circumstances.


UPDATE:

As @NathanCTresh highlighted in his comment, the rule of thumb is indeed an oral tradition. Because this is rule passed on in oral tradition, the term is spoken in modern English ("rule of thumb"), and not in Latin ("pollux regula").

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This is certainly not true at all. Thumb doesn't even have a latin root. –  user20276 Apr 18 '12 at 21:20
    
@Nathan - I'm pretty sure they did not use the literal word "thumb". You'll likely find "pollux regula", if anything. –  Mike Christian Apr 23 '12 at 17:24
    
After re-reading your answer, I see that you're saying it was passed on via oral tradition rather than prescription, and in that light obviously it wouldn't retain the Latin root. I'd like to remove the downvote, if you make a meaningless edit then comment you did I will. :) –  user20276 Apr 23 '12 at 20:37
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Heh, I'm not too concerned about the down vote. However, I appended an update, to highlight your comment. You are very perceptive, to have caught the "thumb" reference. –  Mike Christian Apr 23 '12 at 21:17
    
+1, and thanks. I bet the warning signs were funny. Cavis Canem, inscribed a great deal at Pompei, is I believe the first known Beware of the Dog sign we've discovered. Romans were notorious for signage, so I'm sure the sign quip is true also. –  user20276 Apr 23 '12 at 21:25

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