A rule of thumb is a general method or principle, often applied in a broad sense, that is based on experience or practice rather than theory. It’s a fairly common phrase, but a bit strange when you really think about it. So why exactly do we compare our rules to our thumbs? Where does the saying come from in the first place?
Why Do We Use the Rule of Thumb?
Okay, so maybe you’ve heard this reference to the rule before. That it originated with old English laws that codified and allowed domestic violence to be perpetrated by men against their spouses. Specifically, the law in question stated that a man could beat his wife with a stick–but only if it was no thicker than his own thumb.
While this is a popular myth, there appears to be little (if any) evidence to suggest this rule was ever actually codified into law. Even by the 1600s, this didn’t seem to be a custom for domestic abuse. If anything, abusers probably weren’t going to be taking the time to measure things against their thumbs.
Interestingly enough though, early American courts have made reference to the rule in terms of domestic abuse. While none ever called it a “rule of thumb,” they did reference old laws dictating that wife-beating could only be done with smaller-than-thumb sized implements.
Given that court rulings are based on previous rulings, the rule (though not by name) did kind of make it into laws, albeit uncodified.
Closer to Codification
In 1824, Mississippi courts ruled that domestic violence was legal as long as the weapon was no thicker than the judge’s thumb. We are not at all sure if the judge intended to show up to every instance of domestic violence to offer his thumb, but we don’t really want to try getting into the heads of people who think it’s okay to perpetrate domestic violence.
But again, nobody called it a “rule of thumb.” In fact, the rule of thumb wouldn’t be associated with these not-so-great court rulings until the 1970s.
Printing the Rule of Thumb
The earliest use of the “rule of thumb” in its contemporary form seems to hover around 1685 in the preachings of James Durham.
“Many profest Christians are like to foolish builders, who build by guess, and by rule of thumb (as we use to speak), and not by Square and Rule.”
The rule also makes an appearance in 1690s fencing manuals, dictating that what the combatant does, they do by rule of thumb. Which is probably one of the most apt uses of the phrase when you really stop to think about it.
But latching onto the 1685 definition, it seems that the “rule of thumb” originated as an approximate method of measuring things literally, which then became more figurative.
Like thumbs? Here are a bunch of thumb-words.