A rule of thumb is a general guide for behavior or a quick calculation. Its origins are uncertain, with theories including wife-beating and measurements. The phrase likely originated from a lost form of measurement.
A rule of thumb is defined as a general guide for determining behavior, or a rule that enables a person to make a quick mathematical calculation or remember a formula. It exists to remember something else, general or scientific. In standard usage you could write: A rule of thumb in fashion is never to wear white after Labor Day. You could also substitute “in general” for the phrase.
The proposed origins of the rule of thumb of the phrase are quite creative and sometimes a bit shocking. One theory, advanced by Sharon Fenick in a 1996 newsgroup article, suggests that it applied to the thickness of the stick one might use to beat one’s wife, maid, or child. The evidence for this theory has been disputed, but it is true that English men could legally beat their wives, although the practice was in decline, as noted by William Blackstone in his 1765 work Commentaries on the Laws of England.
Others suggest that the expression may have come from typical measurements made in the absence of rulers. For example, the foot inspired the English and American foot, and an inch may have been used as a form of measurement. In Jonathan Swift’s 1726 novel Gulliver’s Travels, Swift records a rule of thumb for tailors “twice around the thumb is once around the wrist”. So the rule may have originated from a simple form of measurement for tailors when determining the width of shirt cuffs. However, in practice, this measurement is not always accurate.
Some have also suggested that the rule of thumb was a way for brewers to check whether a batch of beer was hot enough. This is also a bit unlikely. The thumb is much less sensitive to heat than a wrist or elbow and would not be a very good measure of temperature. The thumb can be used to measure wind direction, which could be a possible source. You put your thumb in your mouth, lift it in the air, and the side that dries faster gives a direction to the wind.
Most likely, however, the phrase is based on an application of measurement that has been lost to antiquity. Broadly speaking, it is unusual to find new sources that definitively settle the question of the phrase’s origin, but it is certainly not unusual to find many new theories about its original use.
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