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In phonetics and historical linguistics, fusion, or coalescence, is the merger of the features of two segment into one.
A common form of fusion is found in the development of nasal vowels, which frequently become phonemic when final nasal consonants are lost from a language. This occurred in French and Portuguese. Compare the French words un vin blanc [œ̃ vɛ̃ blɑ̃] "a white wine" with their English cognates, one, vine, blank, which retain the n's.
Another example is the development of Greek bous "cow" from Indo-European *gwous. Although *gw was already a single consonant, [ɡʷ], it had two places of articulation, a velar stop ([ɡ]) and labial secondary articulation ([ʷ]). In Greek bous these elements have fused into a purely labial stop [b].
Often the resulting sound has the place of articulation of one of the source sounds and the manner of articulation of the other. An example comes from Malay, where the final consonant of the prefix /məN-/ (where N stands for a "placeless nasal", i.e. a nasal consonant with no specified place of articulation) coalesces with a voiceless plosive at the beginning of the root to which the prefix is attached. The resulting sound is a nasal consonant that has the place of articulation of the root-initial consonant.[1] For example:
An extreme example of fusion occurred in Old Irish, where a vowel fused with a consonant before another consonant. The only feature that remained of the lost consonant was its length, in the form of a long vowel: *[maɡl] → [maːl] "prince". This phenomenon is called compensatory lengthening.
Vowel coalescence is extremely common. The resulting vowel is often long, and either between the two original vowels in vowel space, as in [ai] → [eː] → [e] and [au] → [oː] → [o] in French (compare English day [deɪ] and law [lɔː]), in Hindi (with [ɛː], [ɔː]), and in some varieties of Arabic; or combines features of the vowels, as in [ui] → [yː] → [y] and [oi] → [øː] → [ø].