Percussive Etymology – Rule of Thumb

Percussive Etymology – Rule of Thumb

It’s time again for Percussive Etymology (now sporting its very own header, courtesy of the wife). In this fortnightly feature my trusty hammer and I will explore the origins of one of the many quirky phrases contained in the English language.

Percussive EtymologyThe continued positive response to this feature truly warms my heart. Following the brass monkey shenanigans I have been inundated with suggestions of other phrases to mutilate explain, and even a specific request to once more debunk a popular fallacy regarding the origins of a particular expression. While mythbusting was not part of my initial plan for this feature, I acknowledge that I can provide a valuable service to the linguistic sciences by making this small addition.

In that vein we will today address the expression “rule of thumb”, suggested by Misha Burnett. (By the way, Misha recently published the final instalment in his Book of Lost Doors trilogy, The Worms of Heaven. Why don’t you stop by his blog and give it a look?)

Rule of thumb

The popular belief is that this expression harks back to an old English law, allowing a man to discipline his wife by beating her with a stick, provided that the stick is no thicker than his thumb. While there are several references to such a doctrine cited in early laws of various North American states, there is no documentary evidence that this idea was actually a law in England, and while there are records of judges ruling on husbands giving their wives corporal punishment citing “ancient doctrine” that doctrine is never expressly referred to as a “rule of thumb”.

The true origins of the phrase can be found much further back in history, all the way back to the Roman Empire, in fact. The expression “rule of thumb” is actually an eggcorn of “the ruler’s thumb”, or pollicis princeps in Latin.

A big part of the games in the various circuses in Rome was the gladiator battles. Slaves were brought to the capital from across the empire, and those with military experience and those in the best physical condition were set apart and trained as gladiators, to compete in battles to the death for the glory of Caesar and the entertainment of his subjects.

Sometimes during these battles a combatant would be knocked down or disarmed by his opponent, or he would yield and beg for mercy. The victor would stand over him, weapon poised, an turn his gaze upon Caesar. The crowd would roar their bloodlust as Caesar held out his hand, thumb pointing to the side. Should Caesar point his thumb upwards, the loser lives. However, should Caesar point his thumb down…well, you get the picture.

And so the ruler’s thumb became synonymous with having the final word. Over the centuries, as “ruler’s thumb” morphed into “rule of thumb”, the attached meaning also diluted from having the final word to a general guideline, and that’s how we still use it today.

Of course it is a popular topic of debate among historians whether the Roman Emperors actually used their thumbs in this way to proclaim a death sentence, so this explanation is itself open to question. Consequently I dug around some more, and came across another origin theory.

In the olden days, if one desired to learn a trade like carpentry, masonry or blacksmithing, one had to become an apprentice with a master of the craft. An apprentice would be bound to his master for several years, living in his home and eating at his table, until he qualified as a journeyman.

Once he’d attained that status, the journeyman could charge a fee for his work, but could not employ others nor take an apprentice before completing a master work and getting accepted into the guild as a master craftsman himself.

Gaining admittance to a guild could take very long, and many young men resented having to remain subservient to masters even after completing their apprenticeships. So, they cheated. Some journeymen set off to small, isolated villages and falsely presented themselves as master craftsmen.

When the guilds in the cities found out about this (as they invariably did) the punishment was severe: journeymen found to be in violation of guild rules in this fashion had their right thumbs removed, rendering them incapable of using the tools of their trade. The lucky ones, who did not develop rot in the wound and died as a result, had to become beggars for the remainder of their lives, as no one was willing to invite the wrath of the guilds by employing them.

Over time, this became known as the rule of thumb – a general rule that was observed by all guilds to ensure that members adhered to the standards of excellence which the guilds were established to maintain.


If while reading this you were diligently taking notes so you can share this bit of trivia at your next cocktail party, bless your soul you poor dear, for I’m sorry to inform you that this entire post is a complete fiction (i.e., made up, so please don’t quote me in your dissertation).

However, there really was no English law condoning wife-beating of this nature. The expression rather originated (so most believe) with the practice of carpenters using their thumbs rather than rulers for measurement. This was not exact, but unless one accidentally severed part of one’s thumb (and that’s the closest reality will ever come to my second explanation), it made for a consistent and convenient standard. It could also refer to a method of estimating the depth at which seeds had to be planted for optimal conditions for germination.

If you have a phrase or expression you want me to use for a future edition of Percussive Etymology, tell me in the comments. Until next time.

Images courtesy of The Wife.

8 thoughts on “Percussive Etymology – Rule of Thumb

  1. Great post! There is also a suggestion that the origin of the phrase was related to measurements in woodworking, though as a rule of thumb I tend to take that explanation with – er – pinches of salt… (another thumb-related activity) 🙂 Incidentally, and only mildly off-topic, have you seen ‘Thumb Wars’? As in, Star Wars….with thumbs…

  2. My first response to this very first post that dropped in my RSS Feed Reader from one “H.K” the first (I am assuming) was o_O. After reading your disclaimer I relaxed a bit. I was starting to wonder what Mr 23Thorns had dropped me into here! I see that the thumb can be a most useful appendage when used for good and I am reminded of Stevie-boy (my long suffering husband) and his rule of foot. He refuses to use a tape measure when he assures me he can accurately step out metres. At the end of the day they might not be metres as we know them, but they are reasonably consistent (although that depends on if he fell over a pile of debris in the process of “measuring” or if some nefarious pile of something was directly in the metre foot line methinks…) I have lots of sayings but most of them would make no sense to anyone other than an Aussie so I might keep them to myself. We already have a reputation for being uncouth larrikins and I wouldn’t want to fuel anyone’s fire 😉

    1. I added the disclaimer as apparently I can write nonsense rather convincingly. If only I can get my actual fiction to the same standard. Practise makes perfect, right? I’ve also done that step-measuring move, and also measure “metres” by adding the length of my arm to the width of my shoulders. Hasn’t failed me yet.

      At the moment I’m rather focusing on common English expressions, but it might be a worthwhile exercise at some point to issue an invitation for readers to submit regional expressions for a round of percussive maintenance. Between all the Aussies, Kiwis and Brits that hang out here on occasion we could turn it into a proper party 🙂

  3. Apparently a large number of Indo-European languages use the same word or root for “inch” and “thumb” (French, Italian, Spanish, Sanskrit, Slovac). However, I rather like your wayword blacksmith journeyman.

    1. In my native tongue, Afrikaans (mainly derived from Dutch but also strongly influenced by German, French and Malay), the words for thumb and inch are in fact homonyms, so the measurement angle does appear to be most likely. However, I think my explanation is quite plausible. My ultimate goal is to have one of my theories gain so much ground that it has to be debunked on Snopes.com 😉

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