Compensatory Lengthening

Compensatory lengthening in phonology and historical linguistics is the lengthening of a vowel sound that happens upon the loss of a following consonant, usually in the syllable coda. This may be considered an extreme form of fusion (Crowley 1997:46), or possibly arise from speakers' attempts to preserve a word's moraic count.

An example from the history of English is the lengthening of vowels that happened when the voiceless velar fricative /x/ and its palatal allophone were lost from the language. For example, in Chaucer's time the word night was phonemically /nɪxt/; later the /x/ was lost, but the /ɪ/ was lengthened (and thus changed in quality; see English phonology to /iː/ to compensate. (Later the /iː/ became /aɪ/ by the Great Vowel Shift.)

Both the Germanic spirant law and the Ingvaeonic nasal spirant law show vowel lengthening compensating for the loss of a nasal.

Non-rhotic forms of English have a lengthened vowel before a historical post-vocalic */r/: in Scottish English, girl has a short /ɪ/ followed by a light alveolar /r/, as presumably it did in Middle English; in Southern British English, the */r/ has dropped out of the spoken form and the vowel has become a "long schwa".