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Rule of thumb

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. See also mnemonic.

Origin of the phrase

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. The term is thought to originate with wood workers who used the length of their thumbs rather than rulers for measuring things, cementing its modern use as an inaccurate, but reliable and convenient standard.

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 fully discredited as a hoax. Sir Francis Buller, a British judge, was alleged to have stated that a man may legally beat his wife, provided that he used a stick no thicker than his thumb. However, it is questionable whether Buller ever made such a pronouncement and there is even less evidence that he phrased it as a "rule of thumb"; the rumoured statement was so unpopular that it caused him to be lambasted as "Judge Thumb" in a satirical James Gillray cartoon. The "rule of thumb" was referenced in at least four legal cases from 1782 to 1897, and in each of the known cases it was referred to only to state its invalidity, with one judge calling it "...a barbarous custom which modern authorities condemn. "It's certainly the case that, although British common law once held that it was legal for a man to chastise his wife in moderation, the 'rule of thumb' has never been the law in England. In the modern period, this non-law gained popularity after feminist Del Martin wrote about it in 1976.

Examples of usage

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 (the actual number is 69.3147181 from the natural logarithm of 2, if the percent growth is much much less than 1%). In terms of money, it is frequently easier to use 72 (rather than 70) because it works better in the 4%-10% range where interest rates often lie. 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:

Then they measured my right Thumb, and desired no more; for by a mathematical Computation, that twice round the Thumb is once around the Wrist, and so on to the Neck and Waist, and by the help of my old Shirt, which I displayed on the Ground before them for a Pattern, they fitted me exactly."

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.

Hazardous Material Emergency Response. Hazardous material (hazmat) emergency responders in the US are trained to various levels of capability for action during a hazmat incident, with names for the capabilities varying by locale. Typically, basic-level responders are trained to be able to recognize various types of hazardous materials and establish an isolation perimeter around the site of the incident. Basic-level responders will observe the rule of thumb for establishing a minimum safe distance from a hazmat site. If the responder is uphill and upwind from the incident, and they are able to visually cover the site with a thumb held at arm's length, they are assumed to be a safe distance away from the event. Of course, this varies greatly depending on the nature of the hazmat incident, and should only be used as a general guideline. More specific information about hazardous material isolation is available in a material's material safety data sheet, or in such documents as the Emergency Response Guidebook, published by Chemtrec in the US. Other countries typically have their own hazmat emergency response resources.

Etiquette. In a 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.

Brewing. 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.

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