The Grammarphobia Blog

Rule of thumb

Q: During a recent appearance on WNYC, you tossed off the statement that it’s a myth the phrase “rule of thumb” originated in an English law codifying domestic violence. People working with victims of such abuse frequently use that explanation in building awareness of a HUGE social and moral problem. Instead of debunking this so casually, it would have been helpful if you had taken just a moment to explain what you meant.

A: Thanks for your question, and thanks for being patient. I get such a huge volume of mail that it sometimes takes me weeks to get through it. Only a fraction gets on the blog, but I’m moving you ahead in the queue because this is an important issue.

There are actually TWO myths here. These are the facts.

(1) There was never a law in Britain or the United States, even in “common law,” that allowed a husband to beat his wife with a rod or stick no thicker than his thumb.

(2) The expression “rule of thumb” has no etymological connection with spousal abuse.

In our new book, Origins of the Specious: Myths and Misconceptions of the English Language, which is due out in early May, my husband and I devote a considerable amount of space to these widespread beliefs. I’ll summarize.

The phrase “rule of thumb” entered English in the mid-1600s and referred to a method based on experience or approximation. It first appeared in print, as far as we know, in a sermon by James Durham, a Presbyterian minister in Glasgow.

Durham wrote that “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.”

Durham’s statement implies that “rule of thumb” had existed even earlier. Much later, a 1785 dictionary defined “by rule of thumb” as “to do a thing by dint of practice.”

Etymologists believe the phrase comes from the old custom of using parts of one’s body as rough units of measure.

A man’s foot is about foot long; the palm of the hand is about four inches wide, a unit once called a “hand’s breadth” (a measure still used to gauge the height of a horse); and the thumb is about an inch wide, a unit once called a “thumb’s breadth” and common in the textile trades.

As for the “rule” in “rule of thumb,” think of a ruler or measuring stick.

Now for the legal history. It’s true that a husband once did have the right under English common law to “give his wife moderate correction,” according to Sir William Blackstone’s Commentaries on the Laws of England (1765). But the old right, Blackstone said, began to wane in the 1600s (and thumbs were not in the picture).

Nobody connected thumbs with chastisement until 1782, when an English judge, Francis Buller, supposedly ruled that a husband could beat his wife if the rod or stick were no thicker than his thumb.

There’s no published record of his comments, but the judge was viciously ridiculed in the press and caricatured in cartoons of the day, which labeled him “Judge Thumb” and “Mr. Justice Thumb.” He was fiercely criticized because no such law or precedent existed.

Nevertheless, in the following century, judges in three American court cases – two in North Carolina and one in Mississippi – also referred to such a doctrine.

But none of the judges offered a shred of verifiable evidence that the doctrine had ever existed. (As we now know from legal scholars, it never did.) And none of the judges used the expression “rule of thumb.”

So how did “rule of thumb,” a 17th-century term for a rough measurement, get linked with a mythological legal doctrine?

This seems to have happened after the feminist Del Martin used the phrase, apparently as a pun, in a 1976 report on domestic violence. She presented the debunked legal doctrine as if it were fact, then followed it with her unfortunate play on words.

“For instance,” she wrote, “the common-law doctrine had been modified to allow the husband ‘the right to whip his wife, provided that he used a switch no bigger than his thumb’ – a rule of thumb, so to speak.”

Ever since, we’ve been saddled with both a fictitious legal doctrine and a false etymology.

In case you’d like to know more, here’s the most authoritative study of this issue: Henry Ansgar Kelly, “‘Rule of Thumb’ and the Folklaw of the Husband’s Stick,” Journal of Legal Education, Vol. 44, No.3 (September 1994), pp. 341-365.

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