I have noticed that the diphthong /aʊ/ occurs before certain consonants. We have:

  • /aʊd/ in loud
  • /aʊt/ in out
  • /aʊs/ in house
  • /aʊn/ in town
  • /aʊtʃ/ in pouch
  • /aʊl/ in owl

BUT, we don't have /aʊp/, /aʊb/, /aʊg/, /aʊk/, /aʊm/ (although I only know one word, trauma in which it occurs) etc. Is this a coincidence or is there is a rule for /aʊ/ to occur in certain environments? I guess there's something that needs uncovering but I really don't know what. Can someone please explain this?

  • 1
    How do you pronounce Tolkein's dragon Smaug?
    – Andrew Leach
    Apr 26, 2021 at 10:41
  • @AndrewLeach Good find! I've never heard that before and Wikipedia gives its pronunciation with /aʊg/.
    – user387044
    May 10, 2021 at 10:28

1 Answer 1


I actually wrote a long post about this topic on the Linguistics SE site: you can see it at Why English is missing some phoneme sequences (/aʊv/ or /aʊp/).

Incidentally, trauma is only pronounced with /aʊ/ when it is given a "foreignizing" pronunciation. The anglicized pronunciation uses the thought vowel, the usual pronunciation of the digraph "au" in English.

As you may know, a major source of Modern English /aʊ/ is the Middle English long vowel /uː/, which changed to /aʊ/ as part of the "Great Vowel Shift". For some reason, /uː/ did not undergo this change before a labial consonant (p b m f v): it was either retained as /uː/ (as in room) or in some cases shortened to /ʌ/ or /ʊ/.

The velars are a bit harder to explain. It seems like for /g/, there just weren't many words of the appropriate form to historically lead to a sequence of /aʊg/, but there are some words where shortening seems to have occurred to /uː/ before /k/.

A general pattern seen in English and in some other languages also is that more complexity is allowed in syllables ending with a coronal consonant (d t s n tʃ l all fall in that category) than in syllables ending with another kind of consonant. This more theoretical, abstract point might help explain the pattern of which syllables /aʊ/ is restricted to, although this point doesn't clearly explain why other diphthongs such as /aɪ/ are not restricted the same way (/aɪm/ and /aɪk/ are frequently found in English words).

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    @Sphinx: Yes. Partly because English /ŋ/ developed in consonant clusters, /ŋ/ does not occur after diphthongs in general, except for in special words such as "oink" and "boing".
    – herisson
    Feb 5, 2021 at 9:47
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    I was about to reply from my standpoint of ignorance that the cowpunk browbeat the nowcasting maumauer.
    – JEL
    Feb 5, 2021 at 10:01
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    @JEL: I thought about mentioning that this sort of restriction doesn't apply to compounds, but that seems kind of straightforward to deduce, considering that pretty much any words that can exist separately can be joined into a compound without a change in the constituent phonemes. E.g. the restrictions or mergers that many American speakers have on vowels before /r/ don't apply across compound boundaries either (payroll and keyring don't have the same vowel sounds as pair and peer for me).
    – herisson
    Feb 5, 2021 at 10:06
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    I'd have to check each one (because of anomalies due to special or obsolete pronunciations, for example 'cowcumber'), but for all except, as you say, 'g', there were multiple OED headwords, not all of them compounds. I was just trying to be clever with the sentence (and failing).
    – JEL
    Feb 5, 2021 at 10:09
  • 1
    "Plus boundaries" are different from word or syllable boundaries. Feb 5, 2021 at 16:24

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