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In phonetics, a nasal release is the release of a stop consonant into a nasal. Such sounds are transcribed in the IPA with superscript nasal letters, for example as [tⁿ] in English catnip [ˈkætⁿnɪp]. In English words such as sudden in which historically the tongue made separate contacts with the alveolar ridge for the /d/ and /n/, [ˈsʌdən], many speakers today make only one contact. That is, the /d/ is released directly into the /n/: [ˈsʌdⁿn̩]. While this is a minor phonetic detail in English (in fact, it is commonly transcribed as having no audible release: [ˈkæt̚nɪp], [ˈsʌd̚n̩]), nasal release is more important in some other languages.
Prestopped nasals 
In some languages, such consonants may occur before vowels, and are called prestopped nasals.
Prestopped nasals, and prenasalized stops, occur when the oral cavity is closed, and the nasal cavity is opened by lowering the velum, but the timing of these two events does not coincide. A prenasalized stop starts out with a lowered velum that raises during the occlusion, much like the [nd] in candy. A postnasalized stop or prestopped nasal begins with a raised velum that lowers during the occlusion. This causes an audible nasal release, as in English sudden.
The Slavic languages are most famous for having (non-phonemic) prestopped nasals. This can be seen in place names such as the Dniester River. The Russian word for "day", for example, is inflected день, дня, дни, дней [d̻ʲen̻ʲ, d̻ʲn̻ʲa, d̻ʲn̻ʲi, d̻ʲn̻ʲej], "day, day's, days, days'". (Here the "palatalized" stops are presented as laminal postalveolars.)
Prestopped nasals also found in Australia. The Eastern Arrernte language has both prenasalized stops and prestopped nasals, but does not have word-initial consonant clusters. Compare [mʷaɻə] "good" (with nasal stop), [ᵐbʷaɻə] "make" (with prenasalized stop), [ᵖmʷaɻə] "coolamon" (with prestopped nasal).
Note that there is little or no phonetic difference between a "prenasalized stop", e.g. /ⁿd/, and a cluster, e.g. /nd/, and similarly for prestopped nasals. The difference is essentially one of phonological analysis. For example, languages with word-initial /nd/ (or /ⁿd/), but no (other) word-initial clusters, will often be analyzed as having a unitary prenasalized stop rather than a cluster of nasal + stop. For some languages, it is claimed that a difference exists (often medially) between /ⁿd/ and /nd/. Even in such cases, however, alternative analyses are possible. Ladefoged and Maddieson investigated one such claimed case and concluded that the two sounds were better analyzed as /nd/ and /nnd/, respectively.
Final consonants with nasal release 
However, some languages such as Vietnamese and Malay, which are generally described as having no audible release in final stops, actually have a short nasal release in such cases. Since all final stops in these two languages are voiceless, the nasal release is voiceless as well.
Contrasting releases in Wolof Nasal release Aspirated release [lapᵐ̥] 'to drown' [lapʰ] 'to be thin' [ɡɔkᵑ̊] 'bridle rope' [ɡɔkʰ] 'white chalk'
Final nasal release is also phonemic in colloquial German, in that the extremely common /-ən/ suffix (orthographically -en), used to form plurals (both in nouns and verbs), past participles, and infinitives, is often reduced after a stop to [-ⁿ], causing a nasally-released final stop. For example, the verb sagen, 'to tell, say', is in the first person singular ich sage [ʔɪç zaːgə], but in the plural wir sagen [viːɐ̯ zaːgŋ]. The /ə/ in this suffix is rarely realized, even in careful speech; a syllabic alveolar [n̩] is more common. The reduced form is not syllabic and assimilates to the place of its previous stop, so a word ending in [p] or [b] will have the suffix [m] or [n̩].
- Ladefoged, Peter and Ian Maddieson. The Sounds of the World's Languages.
- Principles of Phonetics, p. 362. John Laver, Cambridge University Press, 1994.