Rule of thumb: check the dictionary

CMAJ 1998;158:1014


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In the article "MDs have key role in bringing ugly secret of wife abuse out of closet" (CMAJ 1997;157[11]:1579-81 [full text / en bref]), by Nicole Baer, I was most perplexed to read the old chestnut that the expression "rule of thumb" is derived from an American law permitting a husband to thrash his wife with a "rattan no wider than his thumb." Although the derivation seems plausible, your readers can be thankful that this macabre yarn is a fabrication, first published in July 1986 in a letter to Ms. magazine from the creative mind of Claire Bride Cozzi. Within only 11 years even that version has evolved: Cozzi cited an undated "English common law" permitting a man to chastise his wife with a "switch" that was to be "no thicker than his thumb."
The true derivation of the term "rule of thumb" has never been in doubt. As the Shorter Oxford English Dictionary on Historical Principles indicates, a rule of thumb is "a method or procedure derived entirely from practice or experience, without any basis in scientific knowledge; a roughly practical method." It first appeared in 1692. In his book Not Guilty, D. Thomas explored the origins and significance of this persistent urban myth.1 As Georges Braque has observed, "Truth exists — only falsehood has to be invented".