Trisyllabic laxing

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 is unstressed:

1. The earliest occurrence of trisyllabic laxing occurred in late Old English and caused stressed long vowels to become shortened before clusters of two consonants when two or more syllables followed.
2. Later in Middle English, the process was expanded to all vowels when two or more syllables followed.
3. The Middle English sound change remained in the language and is still a mostly-productive process in Modern English, detailed in Chomsky and Halle's The Sound Pattern of English.

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" /saʊθ/ vs. "southern" /ˈsʌðərn/. 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" (/oʊˈbiːsɪti/, not */oʊˈbɛsɪti/), although in this case the former was back-formed from the latter in the 19th century.