The English phrase rule of thumb refers to a principle with broad application that is not intended to be strictly accurate or reliable for every situation. It refers to an easily learned and easily applied procedure or standard, based on practical experience rather than theory. This usage of the phrase can be traced back to the seventeenth century and has been associated with various trades where quantities were measured by comparison to the width or length of a thumb.
A modern folk etymology holds that the phrase is derived from the maximum width of a stick allowed for wife-beating under English law, but no such law ever existed. This belief might have originated in a rumored statement by eighteenth-century judge Sir Francis Buller that a man may beat his wife with a stick no wider than his thumb. The rumor produced numerous jokes and satirical cartoons at Buller's expense, but there is no record that he made such a statement.
English jurist Sir William Blackstone wrote in his Commentaries on the Laws of England of an "old law" that once allowed "moderate" beatings by husbands, but he did not mention thumbs or any specific implements. Wife-beating has been officially outlawed for centuries in England and the United States, but continued in practice; several nineteenth-century American court rulings referred to an "ancient doctrine" that the judges believed had allowed husbands to physically punish their wives using implements no thicker than their thumbs.
