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Where did the phrase rule of thumb come from?
A rule of thumb is a practice or procedure that is developed from experience and common sense and has nothing to do with scientific or technical knowledge. It was first used in English around 1692. The expression alludes to making rough estimates of measurements by using one's thumb, the distance to the first knuckle usually being about one inch. The plural form is rules of thumb. There is a story about this phrase's origin involving wife-beating that is quite implausible and not worth going into. Suffice it to say that it is a bit of modern folklore. The real explanation of rule of thumb is that it derives from workers who knew their trade so well that they rarely or never fell back on the use of such things as rulers. Instead they would measure things by, for example, the length of their thumb; they measured, not by a rule(r) of wood, but by rule of thumb. The term was already in metaphorical use by the late 17th century.