The phrase “rule of thumb” might sound a bit strange, but it has nothing to do with rulers or actual thumbs—well, sort of. It’s an old expression that refers to using practical experience or a rough estimate to guide decisions, rather than strict rules or precise measurements.

So where did this curious saying come from? Back in the days before precise measuring tools were widely available, people often used body parts for rough measurements. The thumb, in particular, was a handy (pun intended!) tool. For example, brewers used their thumbs to test the temperature of brewing beer—too hot, and it would kill the yeast; too cold, and the fermentation wouldn’t start. Over time, the thumb became a trusty, informal tool for gauging things, and the phrase “rule of thumb” was born to mean an approximate, practical guideline based on experience.

Contrary to a persistent myth, “rule of thumb” has no historical connection to any old laws about using thumbs for punishment (a story often misattributed to the phrase). It’s really just about using what you have—your thumb—and trusting good old-fashioned know-how to get things done!