In class, I mentioned that the phrase “rule of thumb” reflects a measurement procedure used by tailors (and others) hundreds of years ago, in the absence of modern measuring devices. The rule (as in “ruler”) measures one inch as the distance from the tailor’s knuckle to the tip of their thumb. This is part of an old English system of measurements based on the body that included the yard (which is the distance from the nose to the tip of the index finger) and the foot. Some people in class suggested that the phrase actually stems from an old law that said a man can beat his wife with a stick no wider than his thumb. I have not found any corroborating evidence for this as existing in the common law. There is a little evidence for it in case law (see below).

It should go without saying, but I’ll say it anyway, that in what follows the issue is etymology, not domestic violence. This is not about the prevalence or frequency of wife-beating in historical times, only about the origin of a perfectly good and useful phrase. The story of that origin is part of the heritage of the English language and part of the history of measurement. If anyone reading this knows of other sources that they would like to see linked here, just email the links to me at brown5 –at- uwo.ca (replace –at- with the usual @).

Students who are interested in the origins of the phrase “rule of thumb” might enjoy reading the following:

Mark Israel argues against the “wife-beating” interpretation. He writes:

“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.”
 
In my view, the fact that the French word “pouce” means “thumb” and “inch” is dispositive.
 

Wikipedia:

“The term "rule of thumb" or similar exists in many languages and cultures. Its likely origin is that the thumb is often used for rough measurement by carpenters, seamstresses, and many others. In fact, the measurement of an inch is believed to have been derived from the distance between the tip of the thumb and the first joint.”

Jone Johnson Lewis at About.com argues against the idea that the wife-beating rule was ever coded into English commmon law.

"Rule of thumb" as a phrase predates all such known references, in any case. The "rule of thumb" was used for measurements in many different fields, from brewing to money-changing to art.”

Cecil Adams at The Straight Dope argues, on the basis of research by Christina Hoff Sommers (1994), that there is no mention of the wife-beating rule of thumb in the Blackstone commentaries that are the basis of much of U.S. common law, and that there is no evidence of such a rule in English common law.

“Wife beating has never been legal in the U.S. The Massachusetts Bay Colony prohibited it in 1655, religious groups campaigned against it, and vigilantes occasionally horsewhipped men accused of it. Most states had explicitly outlawed it by 1870.

“The old permissive approach wasn't entirely forgotten, however. It was cited in two court rulings, one in Mississippi in 1824, the other in North Carolina in 1874. Both judges referred to an "ancient law" by which a man was allowed to beat his wife with a stick provided it was no wider than his thumb. Where the judges came up with the thumb angle I don't know; as I say, it is not found in Blackstone. At any rate, both judges rejected the principle--each found the husband guilty in the wife-beating case he was adjudicating. And neither referred to the old law as the rule of thumb.”

For more, see Sommers, C. (1994). Who Stole Feminism? How Women Have Betrayed Women

Jennifer Freyd, a psychology professor at the University of Orgeon, says that her reading of the evidence suggests the rule of thumb for wife-beating was never coded into common law, but was popularly accepted in some places. She adds:

“We caution readers to use restraint in judging others harshly for either their use of the phrase "rule of thumb" or for their pain in hearing the phrase used and believing it refers to domestic violence. Keep in mind that folk etymologies are very often incorrect, and deriding people for a false belief in this area serves little purpose. Most importantly, the problem of domestic violence is truly severe, and our energy is best spent on understanding and preventing domestic violence.”

If you know of any further references on the etymology of this phrase, send me the links.