According to Wiktionary, the word comes:

From French diphtongue, from Ancient Greek δίφθογγος (díphthongos, “two sounds”), from δίς (dís, “twice”) + φθόγγος (phthóngos, “sound”)

Separated into its two logical parts and translated loosely as 'two-sound', it can be compared to any of a variety of other words prefixed with 'di-', such as digraph and diglot, each of which is pronounced with a leading (ironically itself a diphthong) ˈdaɪ, not ˈdɪ.

Why is this word parsed this way? With dissect, for example, it is at least acknowledged that 'dis-sect' is a logical alternative to 'di-s[s]ect', the prevailing pronunciation. With 'diphthong', nobody even seems to ever raise an eyebrow.

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    How else do you mean it could be pronounced? Why would anyone "raise an eyebrow" when it's just natural and logical? Can you please expand? – Kris Feb 25 '19 at 10:56
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    When I say it otherwise, people stare. – Greg Lee Feb 25 '19 at 11:24
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    Then you should also say helico-pter – Hagen von Eitzen Feb 25 '19 at 13:26
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    @Kris As di-phthong, 'dye-fthong'. – aabeba Feb 25 '19 at 14:05
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    It's pronounced the way it's spelled!!! – Hot Licks Feb 25 '19 at 20:34

We break diphthong into syllables differently than the Greeks did. We break it diph-thong, whereas etymologically it is di-phthong. Because there's a consonant on the end of the first syllable, it's natural for English speakers to pronounce it with a "short i", /ɪ/.

The same thing happens with diptych, whose etymology is di+ptykha, where ptykha means folds.

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    Thanks. Care to opine on why dissect is pronounced di-ssect, not dis-sect? Any such rule at work here? – aabeba Feb 25 '19 at 14:09
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    @aabeba Dissect can be pronounced either way, and the short vowel is the original; the diphthongal pronunciation presumably came about due to influence from other words with the /daɪ-/ pronunciation. The prefix dis- (short vowel) in both Latin and Greek became dī- (long vowel) before some voiced consonants, which created side-forms with long and short vowels. The long vowel was diphthongised as part of the Great Vowel Shift in English, leaving two identical prefixes pronounced quite differently; it’s not surprisingly that some confusion would ensue. – Janus Bahs Jacquet Feb 25 '19 at 14:32
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    Slightly off-topic, but along the same lines: we break helicopter as heli-copter, when helico-pter would be truer to the Greek etymology. – gspr Feb 25 '19 at 14:39
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    @aabeba It's not a physiological or practical limitation, because presumably Greek native speakers would have been fine with that syllabication; it has to do with our phonotactics. – Azor Ahai -- he him Feb 25 '19 at 19:28
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    "dissect" comes from Latin "dissecāre" which contains the prefix "dis-". Not "di-", and not from Greek. "dissect" is pronounced with a short i-sound. Any notion that it's pronounced with /dai/ comes from confusion with words that don't contains the prefix "dis-" (possibly words that contain the prefix "di-"). – Rosie F Feb 26 '19 at 11:52

In words from Greek or Latin, a single vowel letter before a consonant cluster that cannot1 occur at the start of a word tends to take its "short"2 pronunciation. The consonant cluster in the middle of "diphthong" cannot come at the start of a word (whether you pronounce it as /fθ/ or as /pθ/), so the "i" in the first syllable is pronounced as /ɪ/.

The converse is not true: before a consonant cluster that can start a word, a single vowel letter may either be "short" or "long". For example, "diploid" is pronounced with /ɪ/ even though there are words that start with /pl/.

As Peter Shor mentioned, the criterion can also be formulated in terms of syllabification. An "open" syllable ends in a vowel, and a "closed" syllable ends in a consonant. In general, we can say that non-final closed syllables in English tend not to have "long vowel" sounds, although that isn't a firm rule of English phonotactics: there are words like "paraleipsis", where the penultimate syllable is closed and pronounced /laɪp/. This is why I included spelling in the rule that I gave at the start of this answer: it would be possible to have a word pronounced /ˈdaɪfθɒŋ/ or /ˈdaɪpθɒŋ/, but it wouldn't be expected to be spelled with diphth-.

There are different approaches to syllabification, but one commonly used criterion says that a consonant cluster can start a syllable only if it can start a word. This criterion implies that consonant clusters that cannot1 occur at the start of a word like /fθ/ always cause the preceding syllable to be closed. Consonant clusters that can occur at the start of a word, like /pl/, would typically be expected to form a syllable onset when the occur word-medially, but some people would argue that the use of /ɪ/ in the first syllable of "diploid" implies the syllabification dip.loid. It's unclear why diploid would be syllabified differently from diglot.

  1. According to the phonotactic rules for typical English words. Exceptions like some people's pronunciation of "phthalate" do not count. Incidentally, some people pronounce words like "psychic" with /ps/, but /ps/ also does not count as "a consonant cluster that can occur at the start of a word" for the purposes of this rule.

  2. "Short" in the phonological, not the phonetic sense: short a e i o u are the phonemes /æ ɛ ɪ ɒ ʌ/. (In American English, /ɑ/ is used instead of /ɒ/.) An alternative term that has been used is "lax".

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  • Phonotactics, eh? Are these firm rules, or merely guidelines that indicate which consonant clusters "should be" impossible for natives of a language to pronounce? Is there a descriptivist front I can sign up for to push for a more progressive English language phonetic system? – aabeba Feb 25 '19 at 14:08
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    The consonant cluster /fθ/ can absolutely appear at the beginning of a word, at least to some people: phthalate/phthalic begins with it, as do phthisis (if you don’t pronounce it /ˈtaɪsɪs/, that is) and the perhaps rather recondite Phthiraptera. – Janus Bahs Jacquet Feb 25 '19 at 14:30
  • But not, say, /pt,/ or /pht/? Pteranodon and pterodactyl, come to mind. And is there any reason initial /ks/ (xylophone, Xerox) still clings to that pesky French 'z' sound? – aabeba Feb 25 '19 at 14:52
  • @aabeba: But isn't the French gzeelofon? – Peter Shor Feb 25 '19 at 21:13
  • @aabeba: The spelling-based "rules" that I talk about in this answer are mostly based on the phonotactics of certain historical stages of English. They aren't necessarily linked to whether English speakers find it impossible to pronounce certain sequences of sounds. – herisson Feb 25 '19 at 21:16

Words that do not originate in English does not necessarily follow English pronunciation rules, And diphthong is no difference as well.

Look at the word dilemma - It is pronounced with dih instead of dahy. https://www.dictionary.com/browse/dilemma?s=t

Now look at the verb divulse - it is pronounces with dahy https://www.dictionary.com/browse/divulse?s=t

But the noun divulsion is pronounced with a dih


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    Actually, dilemma can be pronounced both ways. – Kate Bunting Feb 25 '19 at 12:06
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    What language is this "dahy" pronunciation? It certainly isn't British English. – alephzero Feb 25 '19 at 13:38
  • @alephzero American English, surely? – aabeba Feb 25 '19 at 14:10
  • @aabeba I've never heard of anyone pronouncing dilemma as "day lemma" in the US. Only "die lemma" or "dih lemma". – TylerH Feb 25 '19 at 15:11
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    @TylerH that’s the one I mean—“die-lemma”. – aabeba Feb 25 '19 at 19:03

φθόγγος, diphthongs [this program will not show the lower case φ]

The φ [or lower case version which I can't get here] was sound PH is Ancient Greek.

It started being pronounced as "an aspirated voiceless bilabial plosive, which was the origin of its usual romanization as ." [phytoplankton] [Wikipedia] That is: fee

In modern Greek you get the f sound: dif-thongos where th is theta and i is like ee is fee. (Sorry, can't do the phonemic rendition).

The syllabification in Greek is: δίφ-θόγγος [deef-thongos]but the pronunciation in English is: dip-thong. And the Greek ph as f sound disappears. Only the spelling remains, like the Cheshire Cat's grin. There is no f sound in English in the English pronunciation of the Greek "transliterated" spelling.

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    "Diphthong" may be pronounced in English with either /fθ/ or /pθ/. Dictionaries list both pronunciations. – herisson Feb 25 '19 at 20:25
  • Dictionaries may list it as Wikipedia does A diphthong (/ˈdɪfθɒŋ/ DIF-thong or /ˈdɪpθɒŋ/ DIP-thong. However, if you work with these sounds, no one says: dif-thong. It sounds like a speech impediment that way. There is clearly a syllabic break. Not: di-pthong or di-fthong. – Lambie Feb 25 '19 at 23:02
  • @Lambie: that is a lower case φ. An upper case looks like Φ. There is another way to make a lower case ϕ, but they're really the same letter, like a and ɑ. – Peter Shor Feb 25 '19 at 23:53
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    @PeterShor I know that. I could swear that yesterday, I could not get: φ [or lower case]. I could only get the upper case: Φ. How weird because now it's there. I copied φθόγγος and the φ, LC, came out: Φ, UC. So, now it looks like I don't know the difference and I do. Go figure... – Lambie Feb 26 '19 at 15:01
  • @Lambie That's how I'd do it in the first place. – Kris Feb 27 '19 at 9:37

In English, as in Quantum Mechanics, "Why?" is an invalid question :-).

If 'dipthong' had been regularly pronounced differently then all the current reasons being given would be invalid, no matter how logical, and another set of different explanations of the other pronunciation, equally logical and, in the current situation, wholly incorrect, would be being offered.

The best you can get for an answer is "Because!"

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  • Is it unfair to chart it up to ignorance? In English, a mistake oft repeated ceases to be a mistake, yes, but the acknowledgment at least makes the reality more... palatable. – aabeba Feb 27 '19 at 12:46

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