Place of articulation

In articulatory phonetics, the place of articulation of a consonant is an approximate location along the vocal tract where its production occurs. It is a point where a constriction is made between an active and a passive articulator. Active articulators are organs capable of voluntary movement which create the constriction, while passive articulators are so called because they are normally fixed and are the parts with which an active articulator makes contact. Along with the manner of articulation and phonation, the place of articulation gives the consonant its distinctive sound.
Since vowels are produced with an open vocal tract, the point where their production occurs cannot be easily determined. Therefore, they are not described in terms of a place of articulation but by the relative positions in vowel space. This is mostly dependent on their formant frequencies and less on the specific tongue position and lip rounding.
The terminology used in describing places of articulation has been developed to allow specifying of all theoretically possible contrasts. No known language distinguishes all of the places described in the literature, so less precision is needed to distinguish the sounds of a particular language.

Overview

The human voice produces sounds in the following manner:

Air pressure from the lungs creates a steady flow of air through the trachea, larynx and pharynx. Therefore, the air moves out of the lungs through a coordinated action of the diaphragm, abdominal muscles, chest muscles and rib cage.
The vocal folds in the larynx vibrate, creating fluctuations in air pressure, known as sound waves.
Resonances in the vocal tract modify these waves according to the position and shape of the lips, jaw, tongue, soft palate, and other speech organs, creating formant regions and so different qualities of sonorant sound.
Mouth radiates the sound waves into the environment.
Nasal cavity adds resonance to some sounds such as and to give nasal quality of the so-called nasal consonants.
The larynx

The larynx or voice box is a cylindrical framework of cartilage that serves to anchor the vocal folds. When the muscles of the vocal folds contract, the airflow from the lungs is impeded until the vocal folds are forced apart again by the increasing air pressure from the lungs. The process continues in a periodic cycle that is felt as a vibration. In singing, the vibration frequency of the vocal folds determines the pitch of the sound produced. Voiced phonemes such as the pure vowels are, by definition, distinguished by the buzzing sound of this periodic oscillation of the vocal cords.
The lips of the mouth can be used in a similar way to create a similar sound, as any toddler or trumpeter can demonstrate. A rubber balloon, inflated but not tied off and stretched tightly across the neck produces a squeak or buzz, depending on the tension across the neck and the level of pressure inside the balloon. Similar actions with similar results occur when the vocal cords are contracted or relaxed across the larynx.

Active articulators

The active articulators are movable parts of the vocal apparatus that impede or direct the airstream, typically some part of the tongue or lips. There are five major parts of the vocal tract that move: the lips, the flexible front of the tongue, the body of the tongue, the root of the tongue together with the epiglottis, and the glottis. They are discrete in that they can act independently of each other, and two or more may work together in what is called coarticulation.
The five main active parts can be further divided, as many languages contrast sounds produced within the same major part of the vocal apparatus. The following 9 degrees of active articulatory areas are known to be contrastive :

The lower lip
Various parts of the front of the tongue :
*The tip of the tongue
*The upper front surface of the tongue just behind the tip, called the blade of the tongue
*The surface of the tongue under the tip
The body of the tongue which is sometimes further divided into front and back
The base root of the tongue and the throat
The aryepiglottic fold inside the throat
The glottis at the very back of the windpipe

In bilabial consonants, both lips move so the articulatory gesture brings the lips together, but by convention, the lower lip is said to be active and the upper lip passive. Similarly, in linguolabial consonants the tongue contacts the upper lip with the upper lip actively moving down to meet the tongue; nonetheless, the tongue is conventionally said to be active and the lip passive if for no other reason than that the parts of the mouth below the vocal tract are typically active, and those above the vocal tract are typically passive.
In dorsal gestures, different parts of the body of the tongue contact different parts of the roof of the mouth, but it cannot be independently controlled so they are all subsumed under the term dorsal''. That is unlike coronal gestures involving the front of the tongue, which is more flexible.
The epiglottis may be active, contacting the pharynx, or passive, being contacted by the aryepiglottal folds. Distinctions made in these laryngeal areas are very difficult to observe and are the subject of ongoing investigation, and several still-unidentified combinations are thought possible.
The glottis acts upon itself. There is a sometimes fuzzy line between glottal, aryepiglottal, and epiglottal consonants and phonation, which uses these same areas.

Passive articulators

The passive are the more stationary parts of the vocal tract that the active articulator touches or gets close to; they can be anywhere from the lips, upper teeth, gums, or roof of the mouth to the back of the throat. Although it is a continuum, there are several contrastive areas so languages may distinguish consonants by articulating them in different areas, but few languages contrast two sounds within the same area unless there is some other feature which contrasts as well. The following 9 degrees of passive articulatory areas are known to be contrastive :

The upper lip
The upper teeth, either on the edge of the teeth or inner surface
The alveolar ridge, the gum line just behind the teeth
The back of the alveolar ridge
The hard palate on the roof of the mouth
The soft palate further back on the roof of the mouth
The uvula hanging down at the entrance to the throat
The throat itself, a.k.a. the pharynx
The epiglottis at the entrance to the windpipe, above the voice box

The regions are not strictly separated. For instance, in some sounds in many languages, the surface of the tongue contacts a relatively large area from the back of the upper teeth to the alveolar ridge, which is common enough to have received its own name, . Likewise, the alveolar and post-alveolar regions merge into each other, as do the hard and soft palate, the soft palate and the uvula, and all adjacent regions. Terms like pre-velar, post-velar, and upper vs. lower pharyngeal may be used to specify more precisely where an articulation takes place. However, although a language may contrast pre-velar and post-velar sounds, it does not also contrast them with palatal and uvular sounds so contrasts are limited to the number above, if not always their exact location.

Table of gestures and passive articulators and resulting places of articulation

The following table shows the possible combinations of active and passive articulators.
The possible locations for sibilants as well as non-sibilants to occur are indicated in dashed red. For sibilants, there are additional complications involving tongue shape; see the article on sibilants for a chart of possible articulations.
A precise vocabulary of compounding the two places of articulation is sometimes seen. However, it is usually reduced to the passive articulation, which is generally sufficient. Thus dorsal–palatal, dorsal–velar, and dorsal–uvular are usually just called "palatal", "velar", and "uvular". If there is ambiguity, additional terms have been invented, so subapical–palatal is more commonly called "retroflex".
Note: Additional shades of passive articulation are sometimes specified using pre- or post-, for example prepalatal ; or postvelar. They can be useful in the precise description of sounds that are articulated somewhat farther forward or back than a prototypical consonant; for this purpose, the "fronted" and "retracted" IPA diacritics can be used. However, no additional shade is needed to phonemically distinguish two consonants in a single language.

Homorganic consonants

Consonants that have the same place of articulation, such as the alveolar sounds in English, are said to be homorganic. Similarly, labial and velar are homorganic. A homorganic nasal rule, an instance of assimilation, operates in many languages, where a nasal consonant must be homorganic with a following stop. We see this with English intolerable but implausible; another example is found in Yoruba, where the present tense of ba "hide" is mba "is hiding", while the present of sun "sleep" is nsun "is sleeping".

Central and lateral articulation

The tongue contacts the mouth with a surface that has two dimensions: length and width. So far, only points of articulation along its length have been considered. However, articulation varies along its width as well. When the airstream is directed down the center of the tongue, the consonant is said to be central. If, however, it is deflected off to one side, escaping between the side of the tongue and the side teeth, it is said to be lateral. Nonetheless, for simplicity's sake the place of articulation is assumed to be the point along the length of the tongue, and the consonant may in addition be said to be central or lateral. That is, a consonant may be lateral alveolar, like English , or lateral palatal, like Castilian Spanish ll. Some Indigenous Australian languages contrast dental, alveolar, retroflex, and palatal laterals, and many Native American languages have lateral fricatives and affricates as well.