|Sound change and alternation|
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 [ç] 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.)
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".
Ancient Greek 
Compensatory lengthening is very common in Ancient Greek. It is particularly notable in forms where n or nt comes together with s, y (= ι̯), or i. The development of nt + y was perhaps thus:
- *mont-yă → montsa (palatalization ty → ts) → mõtsa (nasalization and vowel lengthening) → mõssa → mõsa (shortening ss → s) → mōsa (denasalization, retention of long vowel) = μοῦσα "muse"
Forms with this type of compensatory lengthening include the nominative singular and dative plural of many participles, adjectives, and nouns, the 3rd person plural ending for present and future active of all verbs, and the 3rd person singular present of athematic verbs:
- *πάντ-ς → πᾶς "every, whole" (masculine nominative singular)
- *πάντ-ι̯ᾰ → *πάντσα → πᾶσα (feminine)
- *πάντ-σι → πᾶσι (masculine/neuter dative plural)
- compare παντ-ός (m./n. genitive singular)
- *όντ-ι̯ᾰ → *όντσα → οὖσα participle "being" (feminine nominative singular)
- *οντ-ίᾱ → *ονσία → οὐσία "property, essence"
- compare ὀντ-ός (m./n. genitive singular, from participle ὤν "being",)
- Doric φα-ντί → *φαντσί → Attic/Ionic φᾱσί "they say"
See also 
- Crowley, Terry. (1997) An Introduction to Historical Linguistics. 3rd edition. Oxford University Press.
- Smyth, Greek Grammar on CCEL.
- Hayes, Bruce (1989). "Compensatory Lengthening in Moraic Phonology". Linguistic Inquiry (The Massachusetts Institute of Technology) 20 (2): 253–306.
- Millward, C. M. (1996). A Biography of the English Language. Boston: Wadsworth. p. 84.
- Smyth, par. 299: adjs. in nt.
- Smyth, par. 301 a and d: participles in nt.
- Smyth, par. 462 note: Doric athematic verb endings.