Compensatory lengthening

Compensatory lengthening is a sound change in which a vowel or consonant becomes long as a neighboring sound is lost or weakened, often preserving syllable weight or timing.

Mora: A mora is a unit of phonological weight used to measure syllable structure and timing. A lost mora can be reassigned to a neighboring vowel or consonant.
Proto-Indo-European laryngeals: Proto-Indo-European laryngeals are reconstructed consonants whose effects survive in the daughter languages. Their loss lengthened vowels in several Indo-European languages.
Phonemic vowel length: Phonemic vowel length is a contrast in vowel duration that distinguishes words or grammatical forms. A long vowel may be underlying rather than created by neighboring-segment loss.
Phonological quantity: Phonological quantity is the contrastive duration of vowels or consonants in a language. Compensatory lengthening is a historical source of quantity contrasts.
Syllable weight: Syllable weight is a phonological measure, often based on the number of moras in a syllable. Lengthening can preserve a syllable’s weight after a segment disappears.
Hittite language: Hittite is an ancient Anatolian language attested in cuneiform texts from the second millennium BCE. Hittite preserves consonants corresponding to some vowels lengthened elsewhere.
Vowel lengthening: Vowel lengthening is any phonological or phonetic increase in vowel duration. Compensatory lengthening is one historically conditioned kind of vowel lengthening.
Phonological change: Phonological change is a change over time in a language’s sound system or sound patterns. Compensatory lengthening is one recurrent pathway of sound-system change.
Consonant deletion: Consonant deletion is the loss of a consonant from a word or sound sequence. Deleting a consonant is a common trigger for compensatory lengthening.
Ancient Greek phonology: Ancient Greek phonology is the sound system reconstructed for the varieties of Greek spoken in antiquity. Greek long vowels often reflect the loss of Proto-Indo-European laryngeals.

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Osthoff's lawCompared with: It lengthens vowels, the opposite quantity outcome of Osthoff's shortening.