Take for example English voiceless plosives such as /p t k/ which are aspirated at the start of a stressed syllable and before a vowel as in kill, tar, pie:

  • [kʰɪl]
  • [tʰɑː(r)]
  • [pʰaɪ]

But after a preceding /s/:

  • [skɪl]
  • [stɑː(r)]
  • [spaɪ]

the /s/ blocks the aspiration. What does it do to the aspiration? Is there any phonetic explanation?

  • 1
    Most of the air stock gets spent for the pronouncing the sibilant /s/ leaving little for the following plosive, there's even hardly any air left to make the plosion, none remains for the aspiration.
    – Yellow Sky
    Apr 1, 2021 at 10:47
  • 6
    @YellowSky I don't understand your comment (scratches head). A whole utterance of several words is often performed on a single egressive stream of pulmonic air. If what you say held, there'd be no aspiration, or even plosion in any utterance that started with an initial /s/. The other thing is that aspiration is just a period of voiceless vowel before the vocal fold vibration kicks in. It doesn't require an extra reserve of air. And how would the following vowel be performed if there was no air left? Apr 2, 2021 at 22:22
  • @Araucaria-Nothereanymore. - By the air stock I mean the quantity of air needed for the pronunciation of one syllable. Naturally, when we speak and perform utterance of several words on a single egressive stream of pulmonic air, each syllable has a particular properly measured quantity of air intended for its pronunciation, so as while we speak the loudness wouldn't decrease towards the end of the current egressive air stream. I thought it was clear I was speaking about a single syllable, since the /s/ of a previous syllable/word doesn't affect aspiration: [ðis.tʰaʊə] “this tower”.
    – Yellow Sky
    Apr 3, 2021 at 6:49
  • 4
    @YellowSky I still don't quite understand, because even if the /s/ is at the beginning of a syllable, there's a lungful of air available for aspiration should it be required. Apr 4, 2021 at 3:39
  • @Araucaria-Nothereanymore. - Nevermind. Anyhow, I wrote that first comment on April 1. :)
    – Yellow Sky
    Apr 6, 2021 at 8:31

2 Answers 2


Since syllable-final voiceless consonants are also not aspirated ([ɹæt], not *[ɹætʰ]), we generally focus on saying when you get aspiration, and don't say that voiceless stops are intrinsically aspirated. So the rule for assigning aspiration to otherwise unaspirated voiceless stops is that they are aspirated syllable-initially. In skill, /k/ is not syllable initial, so there is no aspiration. It's therefor not about /s/ per se, it's about syllable position and the only thing in English that comes before a stop in a syllable onset is /s/ (or /ʃ/).

There is a problem with the syllable-initial analysis, that there is no aspiration in [ˈhæpi], that is between vowels where the first vowel is stressed and the second is unstressed (cf. also [ˈlɛɪˌtʰɛks]). Either you have to treat "happy" as [ˈhæp.i], or you re-state the rule to refer to the stress foot (a two- or three-syllable unit with stress in the first syllable). People have also tried to relate non-aspiration to physical properties of /s/, e.g. saying that /s/ is aspirated and this is a dissimilation process, but given words like mistrial (two stress feet) blaming the problem on /s/ does not get you far enough.

  • 4
    In some parts of England, word final unvoiced stops are aspirated, sometimes to the degree that /t/ and /k/ are affricated.
    – Colin Fine
    Apr 1, 2021 at 21:39
  • Is there any conclusive evidence that the plosive following an initial /s/ is fortis/voiceless in the first place? Apr 2, 2021 at 22:25
  • @ColinFine Are they aspirated or just lenited? My understanding of aspiration only applies to the voiceless section of a post consonantal vowel. Is there's a different conception of it? Apr 2, 2021 at 22:29
  • 1
    I don't know. Wikipedia says "Aspirated consonants are not always followed by vowels or other voiced sounds. For example, in Eastern Armenian, aspiration is contrastive even word-finally, and aspirated consonants occur in consonant clusters. In Wahgi, consonants are aspirated only when they are in final position" (though that is not sourced). Certainly, Georgian თ is classified as an aspirated consonsant, and the instrumental case-ending -ით ends with it.
    – Colin Fine
    Apr 3, 2021 at 10:42
  • @Araucaria-Nothereanymore. I don’t understand what exactly you mean by “aspirated or just lenited”. The only context I’ve heard those two used about developments in the same ‘direction’ is in relation to Irish (where ‘aspirated’ is now generally avoided since it’s so phonetically inaccurate) – in the context of English, they would be opposites, with aspiration being a kind of strengthening and lenition being weakening. When people pronounce pack as [pʰaʔkʰ] with audible final aspiration in absolute Auslaut, how can that be any kind of lenition? Apr 19, 2021 at 10:33

A bit late to this very interesting question which had also troubled me for years to find the answer. My answer for this question is that it's simply the way those sound are pronounced in English! That is English plosives are unaspirated when they immediately follow /s/ because that's how English speakers do that. So that makes /p/, /t/ and /k/ unaspirated in English /sp/, /st/, /sk/ consonant clusters.

Of course, you can pronounce /sp/, /st/, /sk/ with aspiration, there's no rule telling you that you can't do that and in fact everyone can certainly do that without difficulty. But that's not English /sp/, /st/, /sk/.

In a similar vein, you can also use the same argument to explain why /p/, /t/, /k/ are aspirated when they're at the beginning of a stress syllable.

  • 2
    Very true. One can note here that Classical Greek in fact contrasts st- with sth-, sp- with sph-, and sk- with sch-. Nov 23, 2023 at 10:30
  • Yes. You are giving the complete and normal answer to all questions about language that begin with "why": "Because that's how it is!"
    – Colin Fine
    Nov 23, 2023 at 18:25

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