It seems very common (particularly among non-native English speakers, but I've seen the typed form, though not the spoken form, from native speakers too) for people to type or even say "substraction" when they mean "subtraction".

What causes this error? There must be some loose rule that the word "subtract" fails to follow, to cause the -s- to be inserted so commonly. Is it a phonotactics thing?

I'm not sure if this is the best place to ask this; it's possible that english.SE would be a better place for this. Feel free to move it if so.

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    This isn’t limited to English – as jogloran’s answer mentions, most Romance languages have actually codified the form with the s, and I’ve heard s forms used in German, Swedish and other languages where the standard form has no s, too. I would hypothesise it’s primarily due to influence from ab-, which is abs- before t-. Commented Nov 6, 2023 at 21:35
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    Subst seems rather common in scientific terminology: substitution, substructure, substrate, substance, substantial... I wonder, if this mistake is more typical for people with particular background (e.g., sciences/engineers vs. humanities.)
    – Roger V.
    Commented Nov 7, 2023 at 12:23
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    @RogerV.: Also substage, substellar and substation.
    – dan04
    Commented Nov 7, 2023 at 19:14
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    @RogerV. And not only is 'subst' a common pattern, but the consonant cluster 'str' is quite common, potentially resulting in 'muscle memory' turning 'subtract' into 'substract.' Commented Nov 7, 2023 at 19:24
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    The noun form "substraction" is not common. I had not seen it until I read this post (the closest is substaction). But misspelling of the verb forms are: substract, substracted, and substracting. Where did you sample? Commented Nov 8, 2023 at 23:32

5 Answers 5


The combination bt doesn't appear very often in English (often where it appeared in Latin, it's been simplified in English pronunciation, like "subtle"), while bst appears in words like "abstract". I would chalk this up to analogy with the relatively common and similar-sounding word "abstraction". (In the comments, jogloran also mentions "distraction".)

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    Also extraction, instruction, abstrusion, etc. – in general, there are lots of cases where you have a Latinate prefix and a /tr/ at the beginning of the following root, you get an kind of excrescent /s/ (even though the actual /s/ is only excrescent, other times part of the root or prefix). Commented Nov 6, 2023 at 21:26
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    @JanusBahsJacquet The s in instruction is from the verbal stem struere (cognate to English strew) and not excrescent. Commented Nov 7, 2023 at 9:23
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    @SirCornflakes There was a word missing in my previous comment: it should have said, “even though the actual /s/ is only excrescent sometimes”, alluding precisely to it being part of the root in struere and an inherent part of the prefix in ex-. Commented Nov 7, 2023 at 10:45
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    The absence of real -bt- in English is indeed striking: debt and doubt are further examples of the treatment of this segment in spoken English. On the other hand, pt is quite common and not problematic at all: reptile, crept, ... Commented Nov 7, 2023 at 16:11
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    @SirCornflakes The reason for the silent bs in subtle, debt and doubt is that they all come from old French words that had no b in either spelling or pronunciation (soutil, dette, doute) and the b was added to the spelling by people who recognised Latin words (subtilis, debitum, dubito). This also occured with the silent p in receipt (Old French receite, Latin receptum), but not with the non-p in deceit (Old French deceite, Latin deceptum). Interestingly, the b was restored in French pronunciation for the modern French subtil, but not doute or dette. Commented Nov 8, 2023 at 11:40

I've noticed this too (particularly in non-native but high-level English speakers), but I'm not sure this question is answerable with firm evidence. Different speakers may make the error for different reasons.

If I had to guess, it could be interference from the words "abstraction", "distraction", but note that Spanish actually has sustracción, sustraer, etc, and French has soustraction /sustʁaksjɔ̃/, which might lead speakers of such languages to analogise to *substraction in English.

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    I don't think this is provable.
    – Lambie
    Commented Nov 7, 2023 at 18:02
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    I only started making this mistake after hearing my native Francophone student make it, so there's one anecdotal case! Commented Nov 7, 2023 at 22:02
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    This speech error is made by native Korean speaker June Huh here and native Russian speaker Maxim Kontsevich here, so I wonder if the Francophone influence hypothesis is correct for these examples. In both cases it may not be an on-the-spot error, but rather a misconception that the usual English word really is "substract". Commented Nov 8, 2023 at 13:34
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    @Lambie that's why jogloran wrote "I've noticed" and "If I had to guess".
    – RonJohn
    Commented Nov 8, 2023 at 16:17
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    Native speaker of American English David Malan (professor at Harvard) saying substraction @ 1h19'05"
    – jlliagre
    Commented Nov 11, 2023 at 1:03

Technically, "substraction" is a real word, but it is outdated and considered erroneous in modern English.

However, it derives from the Late Latin "substractus", meaning "take away" and is an alternative form of "subtrahere", which is what "subtraction" is derived from.

According to Wiktionary.org, the usage of "substraction" is common amongst non-native English speakers due to the similarity to the French "soustraction" and the Spanish "sustracción", which both derive from the Late Latin "substractus".

You can also see this related question from English.SE about "substract" vs "subtract".

  • Indeed, it is Medieval Latin. I was surprised to even find a very rare and obolete verb substrahovat(i) in a Czech dictionary, as a variant of rare and obsolete subtrahovat(i). So this dichotomy also exists even outside of Romance languages (+ English). Commented Nov 7, 2023 at 23:10
  • This does not explain why people make the mistake....B English speakers make a lot of other mistakes, too.
    – Lambie
    Commented Nov 8, 2023 at 19:27
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    @Lambie I think that it does explain it to an extent because the etymology of each variation of the word conforms to the rules of English and leads to the same concept. So, an English speaker could feasibly learn "substraction", and it would not feel wrong because it still conforms to the rules of the language.
    – Nelson O
    Commented Nov 8, 2023 at 19:48
  • @NelsonO I have been around (as an interpreter) Portuguese, French and Spanish and I have never ever heard this. Everyone just believes what the want to believe. Cheers.
    – Lambie
    Commented Nov 9, 2023 at 15:22

Others have mentioned that it's likely to be interference from the more common combination st combination found in other words.

But is also seems like it's harder to pronounce bt. Say it slowly and notice the complicated lip and tongue movements that are required. Then insert the s and notice how it smoothes the motion.

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    I don’t think ease of articulation really plays much of a part here, to be honest. Consider obtain and abstain which are mainly distinguished by the presence or absence of the /s/ – but I’ve never heard anyone mix those up. And I’ve certainly never heard anyone say *substerfuge or *substerranean (granted, these have a different stress pattern, which may also have an affect). Commented Nov 7, 2023 at 16:12
  • Good point, although notice that all those examples have a vowel after /bt/.
    – Barmar
    Commented Nov 7, 2023 at 16:46
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    But "bt" isn't pronounced, since the "b" and "t" are in different syllables.
    – RonJohn
    Commented Nov 8, 2023 at 16:28
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    It's interesting to note that obtrusive, the only other word that I could find in which bt occurs at a syllable boundary while followed by a consonant, also seems to trigger /s/ epenthesis frequently enough to warrant a Wiktionary entry for obstrusive as a misspelling of obtrusive.
    – Schmuddi
    Commented Nov 8, 2023 at 18:03
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    But it's not bt: It's sub-tract
    – Lambie
    Commented Nov 8, 2023 at 19:28

I'm not a linguist but I have (unscientifically) noticed as a native American English speaker that many people mispronounce words.

Some words are graced with this more often, with the mispronunciation sometimes even making it into an official source like a dictionary.

Sometimes letters are added, sometimes they're rearranged and sometimes they're removed. The s-sound and letter s seem to be frequent problems, such as with ask (as axe), espresso (as expresso), and even the recent persistent mis-pluralization of words like mom (as moms).

Consider that through the course of a primary education, most native American English speakers are introduced to subtraction (or substraction, as you point out) because it is a core concept in elementary mathematics. They may have cause to use the word often but some combination of society misleading them, difficulty saying certain letter combinations, underdeveloped speaking ability, various speech pathologies, and laziness may cause them to say it incorrectly. Once learned, it may never be corrected.

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    Ax is a simple case of metathesis. The /ks/ variant is generally considered dialectally marked now, but it was just as normal and correct as the /sk/ variant for many centuries – Chaucer and the Tyndale Bible used it, for example. It may have arisen as a mispronunciation, but you can’t classify it as one now, unless you also want to call third and bird ‘mispronunciations’ of thrid and brid. Expresso is a case of analogy (ex- is an existing prefix, es- isn’t). And there is absolutely nothing wrong with pluralising mom as moms. What else would it be? Commented Nov 7, 2023 at 22:35
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    Aside from the tenuous connection to the question, "even the recent persistent mis-pluralization of words like mom (as moms)." — is... "moms" actually a mis-pluralisation?
    – jogloran
    Commented Nov 7, 2023 at 23:58
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    @JanusBahsJacquet Although it's off topic, I believe Stephan Samuel is referring to the use of -s as a hypocoristic suffix, in the specific case of "moms" referring to a single person, as used by black Americans. For example, Christopher Wallace, the Notorious B.I.G., says "I got P-A-I-D, that's why my moms hate me. She was forced to kick me out" (my emphasis). I think "pops" is a better known example of the suffix, and in England we have the terms of address (for one person) "ducks" and "babes", so it's not exclusive to black Americans. Commented Nov 8, 2023 at 11:20
  • "espresso" / "expresso" is covered by "Weird Al" Yankovic (at 1 min 39 secs). Commented Nov 8, 2023 at 23:41

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