Rule of Thumb

I used to have on my desk a daily calendar with tidbits from the Big Book o’ Beer. One entry had, supposedly, the etymology of the expression “Rule of Thumb”.

Adding yeast to beer mix is a tricky procedure. If the mix is too cold, the yeast won’t have enough heat to grow. Too hot, and the yeast will burn up and die. But before brewers had fancy-schmancy tools like thermometers, they had to rely on the most basic of brewing tools: their own thumb. If the mix felt right, the yeast went in. This became known as the “rule of thumb.”

Sounds great. The trouble is that this explanation doesn’t fly. It wasn’t until the late 1800s when Louis Pasteur discovered the existence of yeast. Until then, the agent that caused wort left in open casks to ferment was a mystery. However, thermometers had been common long before then. Like a good geek, I fired up my web browser and began searching for more information on the etymology of the phrase. The most often-encountered “origin” for the phrase “rule of thumb” is a supposed English law that stated that a man could beat his wife with a stick so long as it was no thicker than his thumb. There are just as many refutations of this origin.

The term “rule of thumb” has been used to mean “rough or approximate measure” for several hundred years. This is amply documented in the Oxford English Dictionary and various other works. On the other hand, there are apparently legal cases in both the United States and Great Britain in which judges established a standard for the maximum diamater of a rod with which a man may beat his wife.

According to Wikipedia, this is also the most common, but incorrect, stated origin of the phrase. Wikipedia also notes that the television show “Discoveries This Week” on the Discovery Science Channel in September, 2005 repeated the brewing claim, but also rightly points out

This is doubtful, however, as beer is easily contaminated and ruined by casual contact with unsterilized equipment, much less a thumb.

Further, the wags who participate in the alt.folklore.urban newsgroup fairly thoroughly debunked this in 1996. Not that rules of thumb aren’t helpful (I’m a fan of the 80/20 rule) but the origin of the phrase just isn’t what a lot of people, including the authors of the Big Book o’ Beer, think it is.