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Why is the letter S in the word "sure" is pronounced as "sh"?

If we compare most of the words in English (for instance: surface, surname, sum, sir, soup, sun etc.) then we can easily see that the pronunciation of "sure" is an exception.

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Very interesting question, the reason is in the etymological evolution of spelling and pronunciation of terms and vowels with a French origin.

Here is an extract from The oddest English spellings, part 18: Why sure and sugar? by linguist and etymologist Anatoly Liberman:

  • The vowel occurring in French sure was alien to most Middle English dialects, including the dialect of London, and, as the name of the modern English letter U shows, yu replaced French u in borrowed words.

  • We can observe this substitution even in such a recent loanword as menu (and compare nubile and other nu- words). Once sure appeared in English, it turned into syure, and a similar change happened in sugar (syugar).

  • Later, syu– developed into sh– (compare bless you, session, and Asia, regardless of whether you have a voiced or a voiceless middle in the last of them, for the voicing is secondary).

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    I don't hear many people pronouncing bless as blesh unless they're running their words together.
    – JAB
    Dec 2 '16 at 2:13
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    @JAB I don't think anyone pronounces bless by itself that way, but I've heard people say bless you like that. Dec 2 '16 at 3:46
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    @MattSamuel to use it in a sentence it goes "Ah-tissue - bless you" :D
    – traktor
    Dec 2 '16 at 4:33
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    The French connection is the important point here. Actually, the "syu" sound familiar to English speakers in "bless you" is how you should pronounce sure in French (and the French spelling is sûr).
    – wim
    Dec 2 '16 at 6:04
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    @wim What do you mean? That's not how we pronounce sûr, it's /iu/ instead of /y/ (which does not exist in many many languages) Dec 2 '16 at 8:41
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'Palatalisation' (or 'yod-coalescence') is what caused sure and sugar to have /ʃ/ instead of /s/.

It's a process whereby the clusters /tj/, /dj/, /sj/ and /zj/ coalesce into /t͡ʃ/, /d͡ʒ/, /ʃ/ and /ʒ/, respectively. It often happens when an alveolar obstruent (/t/, /d/, /s/, /z/) is immediately followed by a /j/.

In Modern English, it mostly affects weak syllables for instance the last syllable in 'mission' /ˈmɪ.ʃ(ə)n/.
The letter 'u' used to be (still in some words like 'sue' in some accents) pronounced /juː/ (the same as the word you), but later on it either got dropped (yod-dropping) or coalesced with the following /j/ (yod-coalescence).

The sequence alveolar obstruent + [ju-] in primary stressed syllables simplified either by coalescence or by preservation of the alveolar obstruent and loss of palatality (loss of /j/).

In the sequence [sju-], the only two items in English with stressed stem-initial [sj-] to [ʃ-] change are sure and sugar and their derivatives (in sure and sugar, the /s/ coalesced with the following /j/ and yielded /ʃ/).
The /j/ in other words like super, suit, suet etc., got dropped (historically) in most accents and only the alveolar obstruent /s-/ remained. So super, suit, suet etc., don't have the cluster /sj/ (or /ʃ/).

Other examples of palatalisation include gradual, soldier, passion, nature, elision etc.


/j/ is the sound as in you.
/t͡ʃ/ as in chip.
/d͡ʒ/ as in jam.
/ʒ/ as in elision
/ʃ/ as in ship.

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