November 8, 1996

rule of thumb

I have heard that the origin of the phrase "rule of thumb" comes from an old law that stated a husband could not beat his wife with a stick larger than the diameter of his thumb. The date of this law is unknown.

The date is 1782.

Absolutely amazing, isn't it? This is the sort of explanation that sounds like the worst sort of folk etymology, of after-the-fact explanation. In fact it is a folk etymology, but one that is based on a real fact.

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 first attested in the late seventeenth century, or a hundred years before the thumb-stick-beating practice is first known. (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 between the "rule of thumb," which means 'a general method based on experience or practice as opposed to scientific calculation; a rough or practical method' and the idea of beating one's wife with a thin stick.

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 for 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.