A stop with no audible release, commonly called an unreleased stop, is a stop consonant with no release burst: no audible indication of the end of its occlusion (hold). In the International Phonetic Alphabet, lack of an audible release is denoted with an upper-right corner diacritic (U+031A ◌̚ COMBINING LEFT ANGLE ABOVE) after the consonant letter: [p̚], [t̚], [k̚].
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Audibly released stops, on the other hand, are not normally indicated. If a final stop is aspirated, the aspiration diacritic ⟨◌ʰ⟩ is sufficient to indicate the release. Otherwise, the "unaspirated" diacritic of the Extended IPA may be employed for this: apt [ˈæp̚t˭].
English
In most dialects of English, the first stop of a cluster has no audible release, as in apt [ˈæp̚t], doctor [ˈdɒk̚tər], or logged on [ˌlɒɡ̚dˈɒn]. Although such sounds are frequently described as "unreleased", the reality is that the two consonants overlap so that the release of the first takes place during the hold of the second, masking the first consonant's release and making it inaudible. This can lead to cross-articulations that seem very much like deletions or complete assimilation. For example, hundred pounds may sound like [hʌndɹɨb pʰaundz] but X-ray and electropalatographic studies demonstrate that inaudible and possibly weakened contacts may still be made, so that the second /d/ in hundred pounds does not entirely assimilate a labial place of articulation, but rather co-occurs with it.
Such sounds may occur between vowels, as in some pronunciations of out a lot. The overlap here appears to be with a glottal stop, [t̚ʔ]: the /t/ is pronounced, and since it is between vowels it must be released, but its release is masked by the glottal stop.
The term "unreleased" is also used for a stop before a homorganic nasal, as in catnip. In such cases, however, the stop is released as a nasal, in a nasal release, and so is more precisely transcribed [ˈkætⁿnɪp].
Other languages
In most languages of East and Southeast Asia with final stops, such as Cantonese, Hokkien, Korean, Malay, and Thai, the stops are not audibly released: mak [mak̚]. This is true even between vowels. The effect is thought to be due to an overlapping glottal stop and is more precisely transcribed [mak̚ʔ]. A consequence of an inaudible release is that any aspirated–unaspirated distinction is neutralized. Some languages, such as Vietnamese, which are reported to have unreleased final stops, turn out to have short voiceless nasal releases instead: that is, the excess pressure is released (voicelessly) through the nose, so that there is no audible release to the stop.
Formosan languages
The Formosan languages of Taiwan, such as Tsou and Amis, are unusual in that all obstruents are released but not aspirated, as in Tsou [ˈsip˹tɨ] "four" and [smuˈjuʔ˹tsu] "to pierce", or Amis [tsᵊtsaj] "one" and [sᵊpat˹] "four". (The symbol for a release burst, ⟨˹⟩, is acknowledged but not supported by the IPA.)