There's a phenomenon called Trisyllabic laxing where the vowel in a stressed syllable is shortened if two (or more) syllables follow. If the stressed vowel is in at least* the penultimate syllable (third-last syllable of the word) then it must be lax. It's fairly common in English and mostly occurs when Latin suffixes are attached to a word.


  • Profane, profanity --- [prəˈfn] -> [prəˈfæn.ə.ti]
  • Sincere, sincerity --- [sɪnˈsɪə] -> [sɪnˈser.ə.ti]
  • Impede, impediment --- [ɪmˈpd] -> [ɪmˈped.ɪ.mənt]
  • Divine, divinity --- [dɪˈvʌɪn] -> [dɪˈvɪn.ə.ti]
  • Michael, Michaelmas --- [mʌɪkl] -> [mɪkəlməs]
  • Profound, profundity --- [prəˈfnd] -> [prəˈfʌn.də.ti]
  • Provoke, provocative --- [prəˈvəʊk] -> [prəˈvɒk.ə.tɪv]

The above words have a very regular pattern of vowel change which is summarised below:

  • [eɪ] -> [æ]
  • [ɪə] -> [e]
  • [iː] -> [e]
  • [ʌɪ] -> [ɪ]
  • [aʊ] -> [ʌ]
  • [əʊ] -> [ɒ]

However, in some cases, trisyllabic laxing appears to take place when it should not have done so. For example, Christ [kraɪst] vs Christmas [ˈkrɪs.məs], even though the stress syllable in Christmas is followed by one syllable yet it gets laxed. In such cases, the anomaly is caused by later sound changes. Christmas was a three syllable word (stress on first syllable) when Trisyllabic laxing applied. (Full explanation here)

In some words, it seems to have applied within Latin. That's actually what I'm asking.


  • Decide, decision --- [dɪˈsʌɪd] -> [dɪˈsɪʒ(ə)n], comes from Latin decisionem
  • Collide, collision --- [kəˈlʌɪd] -> [kəˈlɪʒ(ə)n], comes from Latin collisionem
  • Divide, division --- [dɪˈvʌɪd] -> [dɪˈvɪʒ(ə)n], comes from Latin divisionem
  • Reside, residue --- [rɪˈzʌɪd] -> [ˈrɛzɪdjuː], from Latin residuum


  • Did trisyllabic laxing apply to these words within Latin (before entering English)?
  • Is trisyllabic laxing active in Latin?

2 Answers 2


decido and decisio(nem) both have a long i in the second syllable. I do not understand why you think these words are evidence for a "trisyllabic laxing" in Latin.

  • Then why did the vowel in the second syllable get shortened when they entered English? Jul 8, 2020 at 17:55

I'm afraid the answer is no. All these changes happened within English; trisyllabic laxing is a purely English change. For example, the vowel lengths in Latin are dēcīdō and dēcīsiōnem: the relevant vowel is long in both.

The Latin vowel length, note, is very seldom relevant in English. When a Latinate word appears in English, it usually came through Romance (where vowel length was lost centuries ago), or was borrowed in modern times (where most people don't care about Latin vowel lengths).

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