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A rule of thumb is defined as a general guidepost for determining behavior, or a rule that allows a person to make a quick mathematical calculation or remember a formula. It exists to recall something else, whether general or scientific in nature. In standard usage one might write: A rule of thumb in fashion is to never wear white after Labor Day. One might also substitute “in general” for the phrase.
The proposed origins of the phrase rule of thumb are quite creative and sometimes a bit shocking. One theory, posited by Sharon Fenick in a 1996 newsgroup article, suggests that it applied to the thickness of the stick one could use to beat one’s wife, servant or child. The evidence for this theory has been disputed, but it is true that English men could beat their wives legally, though the practice was declining as noted by William Blackstone in his 1765 work Commentaries on the Laws of England.
Others suggest that the expression may have derived from typical measurements made in the absence of rulers. For example, the foot inspired the English and American foot, and a thumb might have been used as a form of measure. 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.” Thus 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 test whether a batch of beer was sufficiently warm enough. This too is a little unlikely. The thumb is far less sensitive to heat than a wrist or an elbow and wouldn’t be a very adequate measure of temperature. The thumb may be used to measure the direction of the wind, which might be a possible source. One puts one’s thumb in the mouth, holds it up in the air, and the side that dries fastest gives one wind direction.
Most likely however, the phrase is based on some measuring application that has been lost to antiquity. As a rule of thumb, it is unusual to find new sources that will definitively solve the question as to the origins of the phrase, but it is certainly not unusual to find many new theories as to its original use.