Rule of thumb

In English, the phrase rule of thumb refers to an approximate method for doing something, based on practical experience rather than theory. This usage of the phrase can be traced back to the 17th 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 relates the phrase to domestic violence via an alleged rule under English common law which permitted wife-beating provided that the implement used was a rod or stick no thicker than a man's thumb. Wife-beating has been officially outlawed in England and the United States for centuries, but enforcement of the law was inconsistent, and wife-beating did continue. However, a rule of thumb permitting wife-beating was never codified in law.

English jurist William Blackstone wrote in the late 1700s in his Commentaries on the Laws of England that, by an "old law", a husband had formerly been justified in using "moderate correction" against his wife but was barred from inflicting serious violence; Blackstone did not mention either thumbs or sticks. According to Blackstone, this custom was in doubt by the late 1600s, and a woman was allowed "security of the peace" against an abusive husband. Twentieth-century legal scholar William L. Prosser wrote that there was "probably no truth to the legend" that a husband was allowed to beat his wife "with a stick no thicker than his thumb".

The association between the thumb and implements of domestic violence can be traced to 1782, when English judge Sir Francis Buller was ridiculed for purportedly stating that a husband could beat his wife, provided that he used a stick no wider than his thumb. There is no record of Buller making such a statement, but the rumor generated much satirical press, with Buller being mocked as "Judge Thumb" in published jokes and cartoons.