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A nasal vowel is a vowel that is produced with a lowering of the velum so that air escapes both through nose as well as the mouth. By contrast, oral vowels are ordinary vowels without this nasalisation.
In most languages, vowels that are adjacent to nasal consonants are produced partially or fully with a lowered velum in a natural process of assimilation and are therefore technically nasal, though few speakers would notice. This is the case in English: vowels preceding nasal consonants are nasalized, but there is no phonemic distinction between nasal and oral vowels (and all vowels are considered phonemically oral). However, the word "huh" is generally pronounced with a nasal vowel.
In French and Portuguese, by contrast, nasal vowels are phonemes distinct from oral vowels, since words that differ mainly in the nasal or oral quality of a vowel exist. For example, the French words beau /bo/ "beautiful" and bon /bõ/ "good" differ only in that the former is oral and the latter is nasal. (To be more precise, the vowel in bon is slightly more open, leading many dictionaries to transcribe it as /ɔ̃/.)
Although loan words exist from French which contain nasal vowels (e.g. "croissant"), there is no expectation that an English speaker would have to nasalize these vowels to the extent French speakers do. Likewise, pronunciation keys in English dictionaries do not indicate nasalization of French loan words.
Suprasegmental and transitional nasal vowels
In Min Chinese, nasal vowels carry persistent air flow though both the mouth and the nose, producing an invariant and sustainable vowel quality. That is, this type of nasalization is synchronic and suprasegmental to the voicing. In contrast, nasal vowels in French or Portuguese are transitional, where the velum ends up constricting the mouth airway.
In languages that have transitional nasal vowels, it is common that there are fewer nasal vowels than oral ones. This appears to be due to a loss of distinctivity caused by the nasal articulation.
Vowel height and nasalization
Nasalization may cause a vowel's articulation to shift. However, while nasalization due to the assimilation of a nasal consonant will tend to cause a raising of the vowel's height, phonemically distinctive nasalization tends to lower the vowel. In most languages, vowels of all heights are nasalized indiscriminately, but preference occurs in some languages, such as to high vowels in Chamorro and low vowels in Thai.
Languages that are written in the Latin alphabet may indicate nasal vowels by a trailing silent n or m, as is the case in French, Portuguese, Bamana, or Yoruba. In other cases they are indicated through diacritics: Portuguese also marks nasality with a tilde, ã, õ, before other vowels; Breton indicates a nasal vowel by a silent trailing ñ with tilde, as in bezañ, "to be"; Polish, Navajo, and Elfdalian use a hook underneath the letter, called an ogonek, as in ą, ę. Other languages may use a superscript n: aⁿ, eⁿ. In the International Phonetic Alphabet, nasal vowels are denoted by a tilde over the symbol for the vowel, as in Portuguese.
The Nasta'liq script used by Urdu denotes nasalization by employing the Arabic letter ˂ن˃ nūn but removing the dot (˂ں˃), called nūn ghunna. Nasalized vowels occur in classical Arabic, but not in contemporary speech or standard Arabic. There is no orthographic way to denote the nasalization, but it is systematically taught as part of the essential rules of tajweed employed while reading the Qur'an. Nasalization usually occurs in recitation when a final ˂ن˃ nūn is followed by a ˂ي˃ yāʼ.
Languages that use phonemic nasal vowels include, among others:
- Austro-Bavarian
- French (see French phonology#Nasal vowels)
- Gbe languages
- Gheg Albanian
- Haitian Creole
- Mandarin Chinese (see Erhua, e.g. (simplified Chinese: 棒儿; traditional Chinese: 棒兒; pinyin: bàngr), (simplified Chinese: 蜂儿; traditional Chinese: 蜂兒; pinyin: fēngr))
- Min Nan (including Taiwanese)
- Paicî (an unusually large number of nasal vowels)
- Polish (most dialects, including Kashubian)
- Tamil (modern colloquial Tamil only; literary Tamil uses oral-vowel plus nasal-stop sequences instead)
- Vietnamese
- Wu (including Shanghainese)
- Yélî Dnye (an unusually large number of nasal vowels)
- Beddor, P. S. 1983. Phonological and phonetic effects of nasalization on vowel height
- The World Atlas of Language Structures Online - Chapter 10 - Vowel Nasalization