Rule of thumb

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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, or in computer science, particularly in algorithm design. See also mnemonic.

Contents

1 Origin of the term

1.0.1 Domestic violence/discipline
1.0.2 Use as a physical measurement
1.0.3 Use in taking temperature
1.0.4 The Russian Tzar
1.0.5 Use to determine wind direction

2 Examples of usage
3 References
4 See also

Origin of the term

The term "rule of thumb" or similar exists in many languages and cultures. Its exact origins are a point of debate as it has such widespread usage that it is hard to determine which usage spawned the term as opposed to the other way around.

Domestic violence/discipline

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. This explanation for the origin of the term was popularized in the opening of the 1999 movie The Boondock Saints.

Caricature condemning Buller

Linguist Michael Quinion, citing the research of Sharon Fenick, notes that there are some examples of a related 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 at all and even less evidence that he phrased it as a "rule of thumb"; 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 English in 1692, long before Buller's reported pronouncement. The first known usage of the phrase "rule of thumb" in direct reference to domestic violence was in 1976, in the book Battered Wives by Del Martin. Martin's usage is figurative, but a literal misreading may have given rise to the misconception of the phrase's origin.

Use as a physical measurement

The measurement of an inch is derived from the distance between the base of the thumbnail and the first joint. This is a plausible origin as the thumb is often used for rough measurement by carpenters, seamstresses, artists and many others.

Quinion notes that this term 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.

Use in taking temperature

Pete's Wicked Trivia, promotional material for Pete's Wicked Ale, states that the phrase came from brewers who used their thumbs to detemine that the beer wort had cooled to the proper temperature for adding yeast.

The Russian Tzar

The Russian Tzar Alexander II has been suggested as the creator of this term. In an apocryphal story he is said to have drawn the route of a railroad from St. Petersburg to Moscow, accidentally tracing his thumb. The engineers, nervous about disobeying his orders, laid the tracks with an unnecessary curve according to the line drawn. It is unlikely this is the true origin of the term, since usage predates this event by several hundred years.

Use to determine wind direction

A technique for approximating the general direction of the wind involves shortly sucking the thumb then raising it in the air. In some languages, the use of the term "rule of thumb" (or equivalent) is used in conjunction with this gesture.

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.

Examples of usage

Financial - 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.

Brewer's Rule 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 adding yeast would stick his thumb into the vat to check the temperature.

References

Michael Quinion on "Rule of Thumb": http://www.worldwidewords.org/qa/qa-rul1.htm
Sharon Fenick at alt.folklore.urban: http://tafkac.org/language/etymology/rule_of_thumb.html
[Rules of Thumb;] some examples.