Trisyllabic laxing, or trisyllabic shortening, is any of three processes in English in which tense vowels (long vowels or diphthongs) become lax (short monophthongs) if they are followed by two syllables, the first of which syllable is unstressed:
The Middle English sound change occurred before the Great Vowel Shift and other changes to the nature of vowels. As a result of the changes, the pairs of vowels related by trisyllabic laxing often bear little resemblance to one another in Modern English; however, originally they always bore a consistent relationship. For example, tense /aʊ/ was [uː], and lax /ʌ/ was [u] at the time of trisyllabic laxing.
In some cases, trisyllabic laxing appears to take place when it should not have done so: for example, in "south" // vs. "southern" //. In such cases, the apparent anomaly is caused by later sound changes: "southern" (formerly southerne) was pronounced [suːðernə] when trisyllabic laxing applied.
In the modern English language, there are systematic exceptions to the process, such as in words ending in -ness: "mindfulness, loneliness". There are also occasional, non-systematic exceptions such as "obese, obesity" (//, not *//), although in this case the former was back-formed from the latter in the 19th century.
|iː||→||ɛ||eː → e
ɛː → e
|eɪ||→||æ||aː → a||profane, profanity;
|aɪ||→||ɪ||iː → i||divine, divinity;
|aʊ||→||ʌ||uː → u||profound, profundity;
south, southern (ME southerne)
|uː||→||ɒ||oː → o||school, scholarly||/skuːl, ˈskɒl.ər.li/|
|oʊ||→||ɒ||ɔː → o||provoke, provocative; sole, solitude||/proʊˈvoʊk, proʊˈvɒk.ə.tɪv/;