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"Decision" is a very common and I have known it for most of the time in my life. It's pronunciation is /dɪˈsɪʒ.ən/. "Decisive" may be common in English speaking countries but I came across it not long ago. When I first saw "decisive", I pronounced the vowel in its second syllable the same as I would pronounce "decision" with, that is, /ɪ/ (the short vowel).

The actual pronunciation of "decisive" is: /dɪˈsaɪ.sɪv/. The second syllable of "decisive" has /aɪ/ and the second syllable of "decision" has /ɪ/. Both are three syllables words. Why is this so? Is there any "reason" for this irregularity?

I can't find anything on Google, I searched many different things but all I get is dictionary definitions of "decision" or "decisive".

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  • It's pronounced the same as the 'i' in 'decide'.
    – Kate Bunting
    Dec 11, 2020 at 13:08
  • This pattern of vowel shortening with I is shared by several other similar word pairs. Most of them are verb–noun pairs: collide/collision, divide/division, elide/elision, incise/incision, provide/provision, recognise/recognition, supervise/supervision, televise/television; but some are adjective–noun: concise/concision, precise/precision; or verb–adjective: suffice/sufficient.
    – Jon Purdy
    Dec 11, 2020 at 19:23

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Good question! It made me do some research.

Compare decide and decision:

  • decide → [dɪˈsaɪd]
  • decision → [dɪˈsɪʒn̩]

'Decide' has a long vowel, 'decision' has short. 'Decisive' also has a long vowel/diphthong even though both 'decision' and 'decisive' are trisyllabic:

  • Decisive → [dɪˈsaɪsɪv]

Here's my best shot:

Decision

According to this answer on ELU, the suffix -ion must have been disyllabic in the past:

  • [di.ˈsi:.zi.ən] (Middle English)

So there are two syllables after the stressed syllable having a long vowel, meaning it's a prime candidate for Trisyllabic Laxing! Trisyllabic Laxing (TSL) was highly productive in Middle English, so it must have been applied to 'decision', shortening the vowel in its second syllable:

  • [di.ˈsi:.zi.ən] → [di.ˈsi.zi.ən] (i: → i)

The Great Vowel Shift didn't affect it because it already had a short vowel.1 (Note, however, that the MidEng TSL applied before the GVS).

Now if you have noticed, there's a glide [j] when we move from a front vowel to another vowel:

  • [zi.ən] → [zjən]

Later on, the [z] and [j] coalesced2 to [ʒ], yielding [dɪˈsɪʒn̩]

Decide

Trisyllabic Laxing didn't affect it because it only had one syllable after the stressed syllable. I don't know much about the history of 'decide'. It was /dɛːˈsiːdən/ in Middle English. The unstressed schwa and terminal nasal were lost in Middle English.

Decisive

It would've been [dɪˈsiːsiv] in Middle English. TSL didn't apply because it only had one syllable after the stressed syllable. Then after the Great Vowel Shift, it became [dɪˈsaɪsɪv].3

I explained it very briefly; there's an awful lot of history to it and I'm still sceptical about my answer. Colin Fine is probably who I'd ask about it.

1. GVS only affected long vowels
2. It's palatalisation
3. The GVS changed [iː] to [aɪ]

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Often a vowel followed by a consonant, followed by an "e" which ends a word will be pronounced as a "long vowel" sound. The same is true for related -er, -ed, and -ing forms of the same roots.

Incision is short vowel, if that helps at all; vision, revision, envision, division - divide, divider - it seems consistent enough to me...

As I native speaker, I never questioned it.

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    Hello Justin. While what you say is a convenient rule of thumb, @Void has pointed out that it doesn't always work; and I'll point out that it is a special case of a more general rule whenever a vowel is followed by single consonant and another vowel: in that position, the syllable was open, so in middle English the vowel tended to be long, and was subsequently affected by the great vowel shirt, as Void says in their answer. The silent e is there because in Middle English it wasn't silent, so those too met the condition I have mentioned.
    – Colin Fine
    Dec 11, 2020 at 21:53

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