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 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.
- Later in Middle English this process was expanded, and applied to all vowels when two or more syllables followed.
- The Middle English sound change remained in the language and is still a mostly productive process in Modern English. This process is detailed in Chomsky & Halle's 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 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/).