• 4-minute read
  • 15th May 2019

Idiom Corner: The Exception That Proves the Rule

“The exception that proves the rule” is a commonly misused phrase in English. Yet it’s also one that most people have heard, so let us clarify how an exception can prove a rule.

The Exception That Proves the Rule

Librarians are notorious food thieves.
Librarians are notorious food thieves.
(Image: Enokson/flickr)

This phrase has its origins in a Latin legal principle that stated “the exception confirms the rule in cases not excepted.” In other words, when there’s an exception to a rule, we know that there must be a rule to which it is an exception (even when this rule isn’t explicit).

For example, if you see a sign saying “No food or drink in the library,” you can work out from this alone that food and drink is allowed in other places. So the exception (i.e., “No food or drink in the library”) proves that another rule must exist (i.e., “Food and drink is permitted outside of the library”).

This is the original use of the phrase and still the “correct” use for many passionate pedants. But it is not what most people now mean by “the exception that proves the rule.” Read on to find out more.

Modern Usage

Old Latin legal principles are not all that popular anymore. Consequently, the phrase “the exception that proves the rule” has taken on a new meaning. Nowadays, then, it usually means the exception that tests the rule.

This is based on a definition of “proves” that we also see in phrases like “proving ground,” “the proof of the pudding is in the eating,” and even in “proofreading.” In all these cases, “proof” means test something to check that it’s valid or correct. As such, an exception can “prove” a rule if it makes us question it (or even reject it). For example, we might believe  “everyone loves pudding” as a rule. But the existence of one person who hates pudding would then be an exception that “proves” or tests this rule.

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Who put the proof in the pudding?
Who put the proof in the pudding?
(Images: Rita E & F=q(E+v^B))

You’ll want to avoid this usage in formal writing, as it is based on a confusion. But people will know what you mean if you use “the exception that proves the rule” this way in conversation.

How Not to Use the Phrase

This phrase is used in another way sometimes: i.e., taking “the exception that proves the rule” to mean an exception can confirm a rule.

Unfortunately, this does not make sense because it involves a direct contradiction. For instance, let us return to the world in which “everyone loves pudding” is a rule. If an exception could “confirm” this, we would have to treat someone who hates pudding as “proof” our original rule was true. And this is clearly absurd, as well as unfair on people who don’t like pudding.

Summary: The Exception That Proves the Rule

To summarize, this phrase has two common uses:

  1. In formal writing, an exception can “prove” the existence of an unstated rule (i.e., if there is an exception to a rule, there must be a rule to which it is an exception). This original use of the phrase is rare in modern English.
  2. The modern use of this phrase is to mean “the exception that tests the rule” (i.e., an exception that makes us question a rule).

However, you should never use this phrase to mean “the exception confirms the rule.” This would be incorrect and illogical. And if you want someone to make sure you’re using idioms correctly, let us know.

Comments (10)
12th August 2020 at 10:59
Call me crazy but I believe the phrase should be "The exception that proofs the rule", as in 'puts it to the test to determine its legitimacy. The name of this site is almost proof of what I'm suggesting. Lol
    12th August 2020 at 11:11
    Thanks for the suggestions, Brian. Unfortunately, "proof" isn't yet a verb (that'd be "proofread" if we're talking about checking writing, or "prove," like in the phrase, for general testing). But maybe if Proofed gets big enough, your version of the phrase will catch on!
      Ben R
      16th November 2022 at 22:20
      Actually, according to Merriam-Webster and American Heritage Dictionary, proof can be a verb.
      19th November 2022 at 11:52
      Hi, Ben. You’re absolutely right! Thank you for pointing that out. One of its meanings in Merriam with this spelling is “to make or take a proof or test of”, as used here, although the spelling “prove” is the one that’s associated with this idiom.
Brenda Dubiel
14th August 2022 at 21:39
I need help. Am I overthinking this library example? I wonder how there can be an exception to this rule when the rule only applies in the library. The conclusion was that there are rules elsewhere, but the sign is specific to the library. I would interpret the exception to the rule to occur in the library rather than outside the library. Help me, please! Thank you.
    27th August 2022 at 14:54
    Hi, Brenda. The library example is demonstrating the original legal usage of the phrase, so here, food and drink being allowed everywhere else is actually the “rule”, while the ban on food and drink in the library is the “exception”. So the exception (i.e., “No food or drink in the library”) proves that another rule must exist (i.e., “Food and drink is permitted outside of the library”). I hope this helps! It is a tricky idiom so it’s easy to see why the alternative modern meaning took over instead.
Brendon Jones
14th November 2022 at 09:49
Perhaps in the original Latin the phrase was easier to understand for a native Latin speaker. Now over time, the modern usage makes more ready sense. Thank you for the explanation, I’ve always struggled with this phrase. I don’t think I learnt many idioms at school.
    19th November 2022 at 11:02
    Hi, Brendon. You're most welcome! I'm glad this article helped :)
26th February 2023 at 21:15
Just listened to a podcast about this phrase. Their example was: "No parking on Sundays", obviously suggesting parking is allowed the rest of the week. I was lead to this web page because I could not really think of any other good examples. Not sure No Food in the Library is a good example, as I would not necessarily assume food is allowed in the classroom, gymnasium, or pool.
    10th March 2023 at 15:48
    Hi, Jim. Thanks for this. Yes, our example of the library is to help explain the original usage of this idiom, but it seems to have fallen out of favor in this sense. We’ve just used a very general idea (of food and drink being allowed everywhere, though of course there are other places that don’t allow food) to help explain what the idiom means, but I think the “No Parking on Sundays” one explains it very well. I hope this is helpful!

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