Trisyllabic laxing

Trisyllabic laxing or trisyllabic shortening is any of three processes in English whereby tense vowels (which are long vowels or diphthongs) become lax (i.e. short monophthongs) in word formation when followed by two syllables, of which the first 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 these changes, the pairs of vowels related by trisyllabic laxing often bear little resemblance to each other 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, for example, in "south" vs. "southern". In such cases, the apparent anomaly is due to later sound changes; e.g. "southern" was pronounced [suːðernə] at the time that trisyllabic laxing applied.

In the modern language, there are systematic exceptions to the process, such as in words ending in -ness (e.g. "mindfulness, loneliness"). There are also occasional, non-systematic exceptions such as "obese, obesity" (/oʊˈbiːsɨti/, not */oʊˈbɛsɨti/).

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