<P> In English, rule of thumb refers to an approximate method for doing something, based on practical experience rather than theory . The exact origin of the phrase is uncertain . Its earliest (1685) appearance in print comes from a posthumously published collection of sermons by Scottish preacher James Durham: "Many profest Christians are like to foolish builders, who build by guess, and by rule of thumb (as we use to speak), and not by Square and Rule". </P> <P> The phrase is also found in Sir William Hope's The Compleat Fencing Master, 1692: "What he doth, he doth by rule of Thumb, and not by Art". James Kelly's The Complete Collection of Scottish Proverbs, 1721, includes: "No Rule so good as Rule of Thumb, if it hit", meaning a practical approximation . </P> <P> Historically, the width of the thumb, or "thumb's breadth", was used as the equivalent of an inch in the cloth trade; similar expressions existed in Latin and French as well . Ebenezer Cobham Brewer writes that in some places the thumb was used to gauge the heat of a brewer's vat . According to Phrasefinder, "The phrase joins the whole nine yards as one that probably derives from some form of measurement but which is unlikely ever to be definitively pinned down". </P> <P> A modern folk etymology relates the phrase to domestic violence via an alleged rule under English law that allowed for wife beating provided the implement used was a rod or stick no thicker than a man's thumb . While wife beating has been officially outlawed in England (and later, the United States) for centuries, enforcement of the law was inconsistent, and wife beating did continue . However, such a rule of thumb was never codified in law . </P>

Where does the term rule of thumb come from