A Good Rule of Thumb
What he doth, he doth by rule of thumb, and not by art.
Sir William Hope. Page 157 of The Compleat Fencing-Master, 1692
And therein lies the earliest known citation of the phrase ‘rule of thumb’, defined by Wikipedia as a ‘principle with broad application that is not intended to be strictly accurate for every situation’.
There are, interestingly, numerous conflicting accounts regarding the origin of the phrase:
- Although it has been said to derive from the belief that English law in the late 1700s allowed a man to beat his wife with a switch as long as it was no thicker than his thumb, the ‘rule of thumb’ has actually never been the law in England and this theory has been fully discredited as nothing more than rumour and hoax. Even The Bias-Free Word Finder, regarded as the bible of politically correct language, considers this origin implausible.
- In the days before thermometers, brewmasters were said to have often gauged the temperature of a batch of wort by dipping a thumb in the brew.
- The Encyclopedia of Word and Phrase Origins indicates that the phrase stems from the ancient use of the last phalange of the thumb of an average adult male as a measuring device for roughly one inch.
Now put that ruler away and get back to work.
David M. Smith