Rule of thumb: Meaning (information, definition, explanation, facts)

A rule of thumb is an easily learned and easily applied procedure for approximately calculating or recalling some value, or for making some determination. Compare this to heuristic, a similar concept used in mathematical discourse, or in computer science, particularly in algorithm design. See also mnemonic.

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. Rules of thumb such as the right hand rule in electromagnetics are also used as mnemonic devices. This usage, of course, is of more recent vintage.

It is widely and erroneously claimed that the term originally referred to the maximum size of a stick with which it was permissible for a man to beat his wife. This claim has been thoroughly debunked, for instance by Christina Hoff Sommers in her book Who Stole Feminism? (1994 ISBN 0684801566). In particular Sommers notes that there is no mention of this in the legal commentaries ofWilliam Blackstone.

The folk etymology of the expression or its urban legend may have received a boost in the following quote from Del Martin of the National Organization for Women:

"In America, early settlers held European attitudes towards women. Our law, based upon the old English common-law doctrines, explicitly permitted wife-beating for correctional purposes. However, certain restrictions did exist and the general trend in the young states was toward declaring wife-beating illegal. For instance, 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"

—Del Martin, Battered Wives Volcano Press, 1976, page 31.

Regardless of whether Martin's analysis of old English common-law doctrines and possibly even her facts, are accurate, there is no evidence that she suggested this usage was the origin of the expression under consideration. However the legend persists. For example:

"Until the 19th Century, there was a charming little rule of thumb that applied to family life. A man was allowed to beat his wife as long as the stick he used was no wider than a thumb."

—Ellen Goodman, Washington Post, April 19, 1983.

"In state courts across the country, wife beating was legal until 1890. There was a rule of thumb, by which courts had stated a man might beat his wife with a switch no thicker than his thumb."

—Chicago Tribune, March 18, 1990.