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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 the nose as well as the mouth. By contrast, oral vowels are vowels without this nasalization. As explained below, nasal vowels that are distinctive or obligatory are of far more linguistic importance than whether or not speakers of a language tend to nasalize vowels in some instances. Relatively similar languages in the same branch of a language family differ on this point quite frequently throughout the world such as in Spanish and Portuguese.
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, but few speakers would notice. That 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 can differ mainly in the nasal or oral quality of a vowel. 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/.)
The Portuguese words rim ("kidney") and ri ((he) "laughs", or (I) "laughed") differ only in that the former's vowel is nasal. Although loan words exist from French that contain nasal vowels ("croissant"), there is no expectation that an English-speaker would have to nasalize these vowels to the extent that French speakers do. Likewise, pronunciation keys in English dictionaries do not always indicate nasalization of French loan words.
Diphthongs can also be nasalized. For example, the Portuguese pronunciation of the city of São Paulo uses the very common nasal diphthong ão (IPA: /ɐ̃w/). Its closest corresponding oral diphthong is au [aw] (found in the word Paulo), and is similar to the English ow, as in now.
Suprasegmental and transitional nasal vowels
In Min Chinese, nasal vowels carry persistent air flow through both the mouth and the nose, producing an invariant and sustainable vowel quality. The nasalization is synchronic and suprasegmental to the voicing. In contrast, nasal vowels in French or Portuguese are transitional, and 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. That appears to be from a loss of distinctivity caused by the nasal articulation.
Vowel height and nasalization
Nasalization may cause a vowel's articulation to shift. However, nasalization from thee assimilation of a nasal consonant tends to cause a raising of vowel height, phonemically distinctive nasalization tends to lower the vowel.
A few languages, such as Palantla Chinantec, contrast lightly nasalized and heavily nasalized vowels. They may be contrasted in print by doubling the IPA diacritic for nasalization: ⟨ẽ⟩ vs ⟨ẽ̃⟩. Bickford & Floyd (2006) combine the tilde with the ogonek: ⟨ẽ⟩ vs ⟨ę̃⟩. (The ogonek is sometimes used in an otherwise IPA transcription to avoid conflict with tone diacritics above the vowels.)
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, Lombard (central classic orthography), 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 under the letter, called an ogonek, as in ą, ę.
Other languages may use a superscript n (aⁿ, eⁿ...), as in the Pe̍h-ōe-jī romanization of Southern Min. 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 occurs in recitation usually when a final ˂ن˃ nūn is followed by a ˂ي˃ yāʼ.
Examles of languages
These languages use phonemic nasal vowels:
- Dutch Low Saxon
- French (see French phonology)
- German (French loanwords of some speakers, Austro-Bavarian and Swabian dialects)
- Gbe languages
- Gheg Albanian
- Haitian Creole
- Malay (Kelantan-Pattani, Terengganu, and Pahang dialects)
- Min Nan (including Taiwanese)
- Paicî (an unusually large number of nasal vowels)
- Polish (most dialects)
- Tamil (modern colloquial Tamil only; literary Tamil uses oral-vowel plus nasal-stop sequences instead)
- Wu (including Shanghainese)
- Yélî Dnye (an unusually large number of nasal vowels)