Rule of thumb
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A rule of thumb is an easily learned and easily applied procedure for approximately calculating or recalling some value, or for making some determination. Compare this to heuristic, a similar concept used in mathematical discourse, or in computer science, particularly in algorithm design. See also mnemonic.
The term "rule of thumb" or similar exists in many languages and cultures. Its likely origin is that the thumb is often used for rough measurement by carpenters, seamstresses, artists and many others. In fact, the measurement of an inch is believed to have been derived from the distance between the tip of the thumb and the first joint. Rules of thumb such as the right hand rule in electrodynamics are also used as mnemonic devices. This usage, of course, is of more recent vintage.
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Origin of the term
It is often claimed that the term originally referred to the maximum size of a stick with which it was permissible for a man to beat his wife, as seen in the following quote from Del Martin of the National Organization for Women:
- "In America, early settlers held European attitudes towards women. Our law, based upon the old English common-law doctrines, explicitly permitted wife-beating for correctional purposes. However, certain restrictions did exist and the general trend in the young states was toward declaring wife-beating illegal. For instance, the common-law doctrine had been modified to allow the husband 'the right to whip his wife provided that he used a switch no bigger than his thumb' -- a rule of thumb, so to speak"
- —Del Martin, Battered Wives Volcano Press, 1976, page 31.
Martin is using the term figuratively ("so to speak"), and is not claiming that the meaning of the phrase itself originates with the common law doctrines. Other examples of the wife-beating origin of the term all follow Martin's use, and are probably influenced by it.
Other examples:
- "Until the 19th Century, there was a charming little rule of thumb that applied to family life. A man was allowed to beat his wife as long as the stick he used was no wider than a thumb."
- "In state courts across the country, wife beating was legal until 1890. There was a rule of thumb, by which courts had stated a man might beat his wife with a switch no thicker than his thumb."
This rumored explanation for the origin of the term was also popularized in the opening of the 1999 movie The Boondock Saints.
Linguist Michael Quinion notes that there are some examples of a similar but not identical usage historically--most notably with regard to a supposed pronouncement by a British judge, Sir Francis Buller. However, it is questionable whether Buller himself made such a pronouncement, and at any rate, the sentiment itself earned Buller opprobrium, causing him to be lambasted as "Judge Thumb" in a satirical James Gillray cartoon.
According to Quinion, the term "Rule of Thumb" was first documented in 1692, long before Buller's reported pronouncement, and is similar to other English phrases tying measurements to parts of the body, for instance the use of the foot, or of the hand as a unit of measurement for horses. Michael Quinion also states that
- Sharon Fenick wrote an article about its origins in the newsgroup alt.folklore.urban in 1996. She found that for more than two centuries there have been references in legal works to the idea that a man may legally beat his wife, provided that he used a stick no thicker than his thumb; but the references were always to what some people believed, not to established legal principle. The British common law had long held that it was legal for a man to chastise his wife in moderation, as one might a servant or child, but Sir William Blackstone wrote in his Commentaries on the Laws of England in 1765 that this principle was by then in decline. So far as I can discover nothing was ever laid down about how such discipline should be applied.
- — Michael Quinion on worldwidewords.com.
According to the Discovery Science Channel's TV show, "Discoveries This Week" on 19 September 2005, the term comes from brewery industry before the advent of thermometers. The man in charge of aiding yeast would stick his thumb into the vat to check the temperature. This is doubtful, however, as beer is easily contaminated and ruined by casual contact with unsterilized equipment, much less a thumb.
Examples
Rule of 72 - a rule of thumb for exponential growth at a constant rate. Divide 72 by the percent interest rate to determine the amount of time to double your money in an investment. For example, at 10% interest, your money will double in 7.2 years (72/10=7.2).
Tailors' Rule of Thumb - A simple approximation that was used by tailors to determine the wrist, neck, and waist circumferences of a person through one single measurement of the circumference of that person's thumb. The rule states, typically, that twice the circumference of a person's thumb is the circumference of their wrist, twice the circumference of the wrist is the circumference of the neck, and twice around the neck is the person's waist. For example, if the circumference of the thumb is 4 inches, then the wrist circumference is 8 inches, the neck is 16 and the waist is 32. An interesting consequence of this is that – for those to whom the rule applies – this simple method can be used to determine if pants will fit: the pants are wrapped around the neck, and if the two ends barely touch, then they will fit. Any overlap or lack thereof corresponds to the pants being too loose or tight, respectively.
References
- Michael Quinion on "Rule of Thumb": http://www.worldwidewords.org/qa/qa-rul1.htm
- Template:Cite web
- Sharon Fenick at alt.folklore.urban: http://tafkac.org/language/etymology/rule_of_thumb.html