I know that English pronunciation is rather arbitrary. There are still some "rules" that even with many exceptions are useful for non-natives like me.

I'm puzzled about the pronunciation of the very common prefix "tri", meaning "three".

"Tri" is often pronounced /traɪ/. For example: triangle, trilateral, tripartite, trioxide, triode, triad, trilobites, triglyceride, triennium, tridimensional, trigeminal, tricycle, tricuspid, triceps, triathlon.

But sometimes "tri" is pronounced /trɪ/. For example: trigamous, triplicate, trio, trinity, trilogy, trillion, trigonometry, triplet, triple, triphthong, triploid, triptych.

Finally, it seems that a few words can be pronounced both ways. For example: trimester, tricolor.

I can find no pattern at all. Is there any "rule"? Or can you help me in any way to guess the right pronunciation of a new word with the prefix "tri"?

  • Not to mention /tree/ as in 'triage'! Commented Feb 13, 2014 at 22:17
  • 1
    Triage comes from the French, not from tri+root
    – Oldcat
    Commented Feb 14, 2014 at 1:18

2 Answers 2


Unfortunately not. As your example words show, you can't apply historical criteria (e.g. 'is it a Greek root') or any obvious phonological decision process ('is it stressed,' 'is there a vowel after it').

The only rule I can think of of is: in chemistry, the pronunciation "tri-" = /traɪ/ is fixed.

I can offer an extension of this rule, which I believe always holds:

  • In a word "triXYZ", if XYZ is itself a word in English, then you pronounce "tri" as /traɪ/.

As an AE speaker, I can't think of a counterexample, anyway.

  • 1
    Your answer is what I was looking for. It seems that actually there is a useful rule, and with few exceptions if any. Applying this rule I suppose that if you create a neologism adding “tri” to an existing word, you pronounce it /traɪ/.
    – Albertus
    Commented Feb 14, 2014 at 16:43

As you say, tri is most commonly pronounced as /traɪ/. But there are some situations where you can predict that you have to use /trɪ/ based on the spelling or the phonological structure of the word.

There are some consonant clusters that can't start a syllable in English: before these, you have to use /trɪ/. This explains /ˈtrɪf.θɑŋ/ and /ˈtrɪp.tɪk/.

Even though doubled consonants are not actually pronounced any differently from single consonants in spoken English, they generally indicate that the preceding vowel is short. This explains /trɪl.jən/.

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