Rules of Thump

From Word Court by Barbara Wallraff in the January-February 2008 Atlantic

Edward Salo, of Mandeville, La., writes: “At work I have noticed my European colleagues making slips with phrases, such as rules of thump for rules of thumb and fuzz factor for fudge factor. These phrases get through e-mail and PowerPoint spell-checkers and sound nearly correct. An Internet search for rules of thump returns numerous entries—is it actually a slip? I hesitate to correct my co-workers, as I am growing rather fond of rules of thump.”

If you search for rules (or rule) of thump in writing vetted by professional editors—for instance, in Google News—you’ll stop doubting it’s a mistake. The phrase is rare in edited media. But please don’t ask me how to explain to your colleagues why the correct expression is rule of thumb.

One popular theory is that it comes from an archaic English law that allowed a man to beat his wife with a stick, provided it was no bigger around than his thumb. If the evidence bore that out, I suppose thump would be nearly as apt as thumb. But this theory has been thoroughly debunked. The expression is more likely to have begun as a reference to measuring with one’s thumb—maybe temperature but more likely size, in which case rule might have originally meant “measuring device” rather than “regulation.”

That is to say, for all that’s known about the origin of rule of thumb, the expression might as well be rule of thump. The reason it’s not is that it’s not.