Stop consonant

First, the oral cavity is closed (hence the name occlusive);
then, it stays closed (hence the names stop), causing pressure to build up;
finally, the closure is suddenly opened; the released airflow produces a sudden impulse in pressure (hence the name plosive) causing an audible sound. This third phase is called release of the stop. In certain languages, final stops may lack the release. In affricate stops, the release simultaneously is a fricative.

Classification of stops

Nasalization

Nasal stops are pronounced with a lowered velum, allowing the air to escape through the nose during the production of the stop; oral stops with a raised velum that shuts the nasal cavity. The term "stop" is often used to refer to oral stops only, with nasal stops called simply nasals. Since nasals are always continuous, not abrupt, it may seem strange to call them stops, though strictly the definition of stops given above allows it.

A prenasalized stop begins with a lowered velum that raises during the second phase of the stop articulation.

A postnasalized stop begins with a raised velum that lowers during the second phase of the stop articulation. This lowering of the velum causes an audible nasal release (e.g. in the English words Edna, sudden).

Note that the terms prenasalization and postnasalization are normally only used in languages where these sounds are not analyzed into sequences of oral stop and nasal stop.

Voice

Voiced stops are articulated with simulaneous vibration of the vocal cords, voiceless stops without.

Aspiration

In aspirated stops, the voice onset (the time when the vocal cords begin to vibrate) comes perceivably later than the release of the stop. The deviation between the release of the stop and the voice onset is called voice onset time (VOT).

Length

In a long stop, the second phase of the articulation of the stop takes more time. In languages where stops are only distinguished by length, the long stops take about three times longer than the short stops. Long stops are often called geminates.

Note that there are many languages where the features voice, aspiration, and length depend on each other (e.g. in English). Because it may be hard to tell which one of these features is predominant, the terms fortis and lenis are sometimes used (in their broader sense).

Airstream mechanism

Stops may be made with more than one airstream mechanism. The normal mechanism is pulmonic, that is with air flowing outward from the lungs. All languages have pulmonic stops. Some languages have stops made with other mechanisms too: these are called ejective, implosive, or click dependent on the mechanism.

Articulatory force

A fortis stop (in the narrow sense) is produced with more muscular tension than a lenis stop (in the narrow sense). This feature has never been proved in measurements.

Examples

Here are some of the oral stops. (The figures in square brackets are from the IPA.)

English stops

[p], [t], [k] (voiceless)

[b], [d], [g] (voiced)

[m], [n], [ŋ] (nasal)

[ʔ] (glottal stop, though not as a phoneme in most dialects)