"rule of thumb"(Phrase Origins)
This term for "a simple principle having wide application but not intended to be strictly accurate" dates from 1692. A frequently repeated story is that "rule of thumb" comes from an old law regulating wife-beating: "if a stick were used, it should not be thicker than a man's thumb." Jesse Sheidlower writes at http://www.randomhouse.com/wotd/index.pperl?date=19961108: "It seems that in 1782 a well-respected English judge named Francis Buller made a public statement that a man had the right to beat his wife as long as the stick was no thicker than his thumb. There was a public outcry, with satirical cartoons in newspapers, and the story still appeared in biographies of Buller written almost a century later. Several legal rulings and books in the late eighteenth and nineteenth centuries mention the practice as something some people believe is true. There are also earlier precedents for the supposed right of a man to beat his wife. "This 'rule' is probably not related to the phrase 'rule of thumb', however. For one thing, the phrase is [...] attested [earlier ...]. (Of course, it's possible that it was a well-known, but unrecorded, practice before Buller.) Another problem is that the phrase 'rule of thumb' is never found in connection with the beating practice until the 1970s. Finally, there is no semantic link [... from what was presumably a very specific distinction to the current sense 'rough guideline']. The precise origin of 'rule of thumb' is not certain, but it seems likely to refer to the thumb as a rough measuring device ('rule' meaning 'ruler' rather than 'regulation'), which is a common practice. The linkage of the phrase to the wife-beating rule appears to be based on a misinterpretation of a 1976 National Organization of Women report, which mentioned the phrase and the practice but did not imply a connection. There is more information about this, with citations from relevant sources, at the Urban Legends Archive." Thumbs were used to measure *lots* of things (the first joint was roughly one inch long before we started growing bigger, and French _pouce_ means both "inch" and "thumb"). The phrase may also come from ancient brewmasters' dipping their thumb in the brew to test the temperature of a batch; or from a guideline for tailors: "Twice around the thumb is once around the wrist..." For a definitive rule of thumb, see the paper "Thumb's rule tested: Visual angle of thumb's width is about 2 deg." by Robert P. O'Shea in _Perception_, 20, 1991, pp. 415-418.
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