Trisyllabic laxing

Trisyllabic laxing, or trisyllabic shortening, is a process in English in which tense vowels (long vowels or diphthongs) become lax (short monophthongs) if they are followed by two syllables, and the first of the following syllables is unstressed. This generally involves words from Latin or Greek, particularly when suffixes are added, causing the tense vowel to become lax. It is complicated by the Great Vowel Shift, which caused tense vowels in stressed syllables to undergo significant changes, so that the pairing of tense and lax vowels in trisyllabic laxing do not seem phonologically intuitive, especially for those learning English as a second language.

1 Basic patterns

Examples of the patterns are as follows.

Tense vowel	→	Lax vowel	Example	Phonetic transcription
iː	→	ɛ	serene, serenity; impede, impediment	/sɪˈriːn, sɪˈrɛn.ɪ.ti/; /ɪmˈpiːd, ɪmˈpɛd.ɪ.mənt/
eɪ	→	æ	profane, profanity; grateful, gratitude	/proʊˈfeɪn, proʊˈfæn.ɪ.ti/ /ˈɡreɪt.fəl, ˈɡræt.ɪ.tjuːd/
aɪ	→	ɪ	divine, divinity; derive, derivative	/dɪˈvaɪn, dɪˈvɪn.ɪ.ti/; /dɪˈraɪv, dɪˈrɪv.ə.tɪv/
aʊ	→	ʌ	profound, profundity; pronounce, pronunciation; south, southern	/proʊˈfaʊnd, proʊˈfʌn.dɪ.ti/; /proʊˈnaʊns, proʊˌnʌn.siˈeɪ.ʃən/; /saʊθ, ˈsʌð.ərn/
uː	→	α / ɔ	school, scholarly	/skuːl, ˈskαl.ər.li/
oʊ	→	α / ɔ	provoke, provocative; sole, solitude	/proʊˈvoʊk, proʊˈvαk.ə.tɪv/; /soʊl, ˈsαl.ɪ.tjuːd/

Note: Whether one shortens the /uː/ or /oʊ/ to /ɒ/, /ɔ/, /α/, or even /a/ depends on dialect or one's variety of English.