rule of thumb

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Look up rule of thumb in Wiktionary, the free dictionary.

A rule of thumb is a principle with broad application that is not intended to be strictly accurate or reliable for every situation. It 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, psychology and computer science, particularly in algorithm design.

Origin of the phrase

The exact origin of the phrase is uncertain, either it is derived from the use of the thumb as a measurement device ("rule"), or it is derived from use of the thumb in a number of apocryphal "rules" (law, principle, regulation, or maxim). The earliest citation comes from Sir William Hope’s The Compleat Fencing-Master, second edition, 1692, page 157: "What he doth, he doth by rule of thumb, and not by art."^[1][2] The phrase also exists in other languages, for example Swedish Tumregel, Norwegian Tommelfingerregel, sometimes in the variant "rule of fist", for example German Faustregel or Dutch Vuistregel. This suggests that it has some antiquity, and does not originate in specifically English-language culture.

Thumb as measurement device

The term is thought to originate with wood workers who used the width of their thumbs rather than rulers for measuring things, cementing its modern use as an imprecise yet reliable and convenient standard.^[3] This sense of thumb as a unit of measure also appears in Dutch, in which the word for thumb, duim, also means inch.^[4] The use of a single word or cognate for "inch" and "thumb" is common in many other Indo-European languages, for example, French: pouce inch/thumb; Italian: pollice inch/thumb; Spanish: pulgada inch, pulgar thumb; Portuguese: polegada inch, polegar thumb; Swedish: tum inch, tumme thumb; Sanskrit: angulam inch, anguli finger; Slovak: palec inch/thumb.

Another possible origin of the phrase comes from measurement, particularly in agricultural fields. The plants need a fairly precise depth to seed properly, whether planted from seed or being replanted, but the depth can sometimes be estimated using the thumb. That is, a rule "(measurement) of thumb." According to Gary Martin, "The origin of the phrase remains unknown. It is likely that it refers to one of the numerous ways that thumbs have been used to estimate things - judging the alignment or distance of an object by holding the thumb in one's eye-line, the temperature of brews of beer, measurement of an inch from the joint to the nail to the tip, or across the thumb, etc. The phrase joins the whole nine yards as one that probably derives from some form of measurement but which is unlikely ever to be definitively pinned down."^[5] One derived use of the thumb as a meaure is in formal place setting, the silverware and the dinner plate should be set back from the edge of the table a length equal to the distal phalanx of the thumb.^{[citation needed]}

Thumb used for regulation

Caricature condemning Buller: Judge Thumb - Patent Sticks for Family Correction - Warranted Lawful!

It is often claimed that the term originally referred to a law that limited the maximum thickness of a stick with which it was permissible for a man to beat his wife, but this has been discredited.^[1][3] British common law before the reign of Charles II permitted a man to give his wife "moderate correction", but no 'rule of thumb' (whether called by this name or not) has ever been the law in England.^[6][7] However, "moderate correction" specifically excluded beatings, only allowing the husband to confine a wife to the household.^[8]

Nonetheless, belief in the existence of a "rule of thumb" law to excuse spousal abuse can be traced as far back as 1782, the year that James Gillray published his satirical cartoon Judge Thumb. The cartoon lambastes Sir Francis Buller, a British judge, for allegedly ruling that a man may legally beat his wife, provided that he used a stick no thicker than his thumb, which may have occurred but poor recordkeeping for trial transcripts in that era make such a ruling extremely difficult to document. In the United States, legal decisions in Mississippi (1824) and North Carolina (1868 and 1874) make reference to—and reject—an unnamed "old doctrine" or "ancient law" by which a man was allowed to beat his wife with a stick no wider than his thumb.^[1] In 1976, feminist Del Martin used the phrase "rule of thumb" to describe such a doctrine, and the usage gained currency in 1982, when the U.S. Commission on Civil Rights issued a report on wife abuse, titled "Under the Rule of Thumb."^[7] No law in the United States has ever permitted the beating of women. The Body of Liberties adopted in 1641 by the Massachusetts Bay colonists states, “Every married woman shall be free from bodily correction or stripes by her husband, unless it be in his own defense from her assault.” ^[9]

Another example comes from marine navigation. A ship's captain should navigate to keep the ship more than a thumb's width from the shore, as shown on the nautical chart being used. Thus, with a coarse scale chart, that provides few details of nearshore hazards such as rocks, a thumb's width would represent a great distance, and the ship would be steered far from shore; whereas on a fine scale chart, in which more detail is provided, a ship could be brought closer to shore.^{[citation needed]} Brewing also has a rule of thumb, before the invention of thermometers, the brewer tested the wort by placing his thumb in it. When he could reliably place his thumb in the wort without having to remove it because of the heat, the wort was cool enough to pitch the yeast.^{[citation needed]}

Examples of usage

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Financial - Rule of 72. A rule of thumb for exponential growth at a constant rate. An approximation of the doubling time formula used in population growth, which says divide 70 by the percent growth rate (using continuous compounding, the actual number would be 69.3147181 or 100 times the natural logarithm of 2). In terms of money, since we use the annual effective interest rate (which is equivalent to annual compounding) for interest rates between 4% and 12% the number which gives the most accurate result is actually 72. Therefore, divide 72 by the percent interest rate to determine the approximate amount of time to double your money in an investment. For example, at 8% interest, your money will double in approximately 9 years (72/8 = 9).

Tailors' Rule of Thumb. This is the fictional rule described by Jonathan Swift in his satirical novel Gulliver's Travels:

See also

• Heuristic argument
• Common sense
• Back-of-the-envelope calculation

References

Further reading

Henry Ansgar Kelly, ‘Rule of Thumb and the Folklaw of the Husband’s Stick’, Journal of Legal Education, 44.3 (September 1994), 341-65.

External links

• Every rule of thumb on earth in one place - A user participation reference project based on Tom Parker's 3 best-selling books and more than 25 years of research.
• A collection of various Rules of Thumb - A wiki based collection of user submitted rules of thumb
• Straight Dope - Discussion of the origins of the term by Cecil Adams.
• Business Rules of Thumb . com - A collection of business-oriented rules of thumb
• Debunker.com - Article debunking the "Rule of Thumb" connection to wife-beating
• European Men Profeminist Network - Faulty legal citations of the bogus wife-beating law.