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A stop, plosive, or occlusive is a consonant sound produced by stopping the airflow in the vocal tract. Plosives are properly stops with airflow out of the mouth. Sometimes the term stop includes nasal stops, more commonly simply called nasals, in which airflow is stopped in the mouth, but released through the nose.
The terms plosive and stop are usually used interchangeably, but they are not perfect synonyms. Plosives are a subset of stops, oral stops with a pulmonic egressive airstream mechanism. That is, airflow is released outwards through the mouth. This contrasts with implosive consonants, where airflow is sucked in.
The term is also used to describe nasal (non-oral) stops (sounds like [n] and [m]). Many use the term nasal continuant rather than nasal stop to refer to sounds like [n] and [m]. This article treats these "nasal continuants" as nasal stops (usually simply nasal consonant), where airflow is stopped in the mouth, but proceeds out through the nose.
In Ancient Greek, stops were called áphōna (stoicheîa), which was translated into Latin as mūtae or surdae (cōnsōnantēs). Both mute and surd were used in English as synonyms of "stop". Both the Latin and Greek terms sometimes referred to consonants in general, which ancient grammarians did not consider as pronounceable on their own without vowels.
All languages in the world have stops and most have at least [p], [t], [k], [n], and [m]. However, there are exceptions: Colloquial Samoan lacks the coronals [t] and [n], and several North American languages, such as the northern Iroquoian languages, lack the labials [p] and [m]. In fact, the labial stop is the least stable of the voiceless stops in the languages of the world, as the unconditioned sound change [p] > [f] (> [h], Ø) is quite common in unrelated languages, having occurred in the history of Classical Arabic, for instance. Some of the Chimakuan, Salishan, and Wakashan languages near Puget Sound lack nasal stops [m] and [n], as does the Rotokas language of Papua New Guinea. In some African and South American languages, nasal stops occur, but only in the environment of nasal vowels, and so are not distinctive. Formal Samoan has only one word with velar [k], but it has a nasal velar stop, [ŋ]. Ni‘ihau Hawaiian has [t] for /k/ to a greater extent than Standard Hawaiian, but neither distinguish a /k/ from a /t/. It may be more accurate to say that Hawaiian and colloquial Samoan do not distinguish velar and coronal stops than to say they lack one or the other.
In the articulation of the stop, three phases can be distinguished:
In aspirated stops, the vocal cords (or vocal folds) are abducted at the time of release. In a prevocalic aspirated stop (a stop followed by a vowel or sonorant) the time when the vocal cords begin to vibrate will be delayed until the vocal folds come together enough for voicing to begin, and will usually start with breathy voicing. The duration between the release of the stop and the voice onset is called the aspiration interval or sometimes the voice onset time (VOT). In Tenuis stops the vocal cords come together for voicing immediately following the release and there is little or no aspiration (a voice onset time close to zero). There may be a brief segment of breathy voice which identifies the stop as voiceless and not voiced. In voiced stops, the vocal folds are set for voice before the release, and the voicing is not initially breathy. Often, but not always, there is voicing during the period of occlusion of a voiced stop. A stop is called "fully voiced" if it is voiced during the entire occlusion. In English, however, initial voiced plosives like /b/ or /d/ may have no voicing during the period of occlusion, or the voicing may start shortly before the release and continue after release. Highly aspirated stops have a long period of aspiration, so that there is a long period of voiceless airflow (a phonetic [h]) before the onset of the vowel.
In most dialects of English, the final voiced stop g in the word bag is likely to be fully voiced, while the occlusion for the initial voiced stop b will be only partially voiced. Initial voiceless plosives, like the p in pie, are aspirated, with a palpable puff of air upon release, while a plosive after an s, as in spy, is tenuous (unaspirated). When spoken near a candle flame, the flame will flicker more after the words par, tar, and car are articulated, compared with spar, star, and scar. In the common pronunciation of “papa”, the initial p is aspirated while the medial p is not.
In a geminate or long stop, the occlusion lasts longer than in normal stops. In languages where stops are only distinguished by length (e.g. Arabic, Ilwana, Icelandic), the long stops may last up to three times as long as the short stops. Italian is well known for its geminate stop, as the double t in the name Vittoria takes just as long to say as the ct does in English Victoria. Japanese also prominently features the geminate consonant, such as in the minimal pair 来た (kita), meaning came, and 切った (kitta) meaning cut (past).
Note that there are many languages where the features voice, aspiration, and length reinforce each other, and in such cases it may be hard to tell which of these features predominates. In such cases the terms fortis is sometimes used for aspiration or gemination, while lenis is used for single, tenuous or voiced stops. Be aware, however, that the terms fortis and lenis are poorly defined, and their meanings vary from source to source.
A prenasalized stop starts out with a lowered velum that raises during the occlusion. The closest examples in English are consonant clusters such as the [nd] in candy, but many languages have prenasalized stops that function phonologically as single consonants. Swahili is well known for having words whose spellings begin with mp or nd, like mtu, though truer prenasalized sounds like [mp] or [nd] do occur word-initially in other Bantu languages.
A postnasalized stop begins with a raised velum that lowers during the occlusion. This causes an audible nasal release, as in English sudden. Russian and other Slavic languages have words that begin with [dn], which can be seen in the name of the Dnieper River.
Note that the terms prenasalization and postnasalization are normally only used in languages where these sounds are phonemic, that is, not analyzed into sequences of plosive plus nasal stop.
Stops may be made with more than one airstream mechanism. The normal mechanism is pulmonic egressive, that is, with air flowing outward from the lungs. All languages have pulmonic stops. Some languages have stops made with other mechanisms as well: ejective stops (glottalic egressive), implosive stops (glottalic ingressive), or click consonants (velaric ingressive).
A fortis stop (in the narrow sense) is produced with more muscular tension than a lenis stop (in the narrow sense). However, this is difficult to measure, and there is usually debate over the actual mechanism of alleged fortis or lenis consonants.
There are a series of stops in Korean, sometimes written with the IPA symbol for ejectives, which are produced using "stiff voice", meaning there is increased contraction of the glottis than for normal production of voiceless stops. The indirect evidence for stiff voice is in the following vowels, which have a higher fundamental frequency than those following other stops. The higher frequency is explained as a result of the glottis being tense. Other such phonation types include breathy voice, or murmur; slack voice; and creaky voice.
[p], [t], [k] (aspirated word-initially, tenuis in clusters with s)
[b], [d], [ɡ] (in most dialects: partially voiced word-initially, fully voiced intervocally)
[ʔ] (glottal stop, not as a phoneme in most dialects)