| In phonetics, the point in the mouth or throat at which a consonant is made. With a vowel, the tongue sits in the centre of the mouth, and its shape determines the shape of the resonance, but with consonants the tongue or some other organ forms either a blockage or a constriction at some definite point between the lips and the larynx.
The basic classification of speech sounds by place of articulation (also called point of articulation) is often shown on a diagram called a sagittal section, a cross-section of the human head. From left to right, the places of articulation are:
Any given language is unlikely to have sounds made at all of the above positions, but typically many if not most of them are used. For example, English has these sounds:
- bilabial: p b m
- labiodental: f v
- dental: th
- alveolar: t d n l s z
- postalveolar: sh ch j r
- palatal: y
- velar: k g ng
- glottal: h
We can make more detailed distinctions, for example between interdental (tongue between the teeth) and apico-dental (tip of the tongue against the upper teeth) and lamino-dental (blade of the tongue against the upper teeth), but no one language is likely to have both interdental and lamino-dental sounds, so they are usually called just dental, with a brief mention of the precise articulation.
A small number of sounds have more than one place of articulation: the English w and the common West African sounds kp gb are labiovelar, that is simultaneously bilabial and velar.
A lot of languages have some kind of secondary articulation, with a primary articulation at one place and a "colouring" by a weaker articulation at another. The English l is purely alveolar at the beginning of a syllable, as in law, but at the end of a syllable as in all it also has a secondary velar quality. It is described as velarized. In Russian a lot of consonants occur either velarized or palatalized. Irish and Polish also have a range of palatalized versus non-palatalized sounds. In Arabic several consonants are pharyngealized. Other languages have a secondary set of labialized sounds. The North Caucasian languages are especially rich in multiple articulations.
Many other features in addition to place make consonants what they are: voice (vibration of the larynx), opening of the passage into the nasal cavity, degree of stoppage or friction, direction of air flow, and so on. One such feature is the bending back of the tongue at the postalveolar position: this is called retroflex and is usually listed as if a separate place of articulation, though strictly speaking it's not. American English r is usually retroflex. Most Indian languages have retroflex t d n l.
Some sounds are easier to make than others. The uvular and pharyngeal articulations are rare across the world. Some articulations don't occur: tongue against the lower teeth, for example, or a reverse labiodental with upper lip against lower teeth. (This does occur as a speech defect.)
But it is unwise to say a sound never occurs. In recent years tribes in Solomon Islands and Vanuatu have thrown up surprises, a couple of languages having linguo-labial sounds (tongue against the lips), and others with a bilabial roll (the brr sound we say when we're cold). Some extraordinarily difficult uvular sounds have been reported from Guatemalan languages. Recent study has shown that the pharyngal sounds of Arabic are in many speakers in fact epiglottal. When I read this I had to go to the library to read up Gray's Anatomy to find out where on earth precisely the epiglottis was and how you could move it. The chief danger of studying linguistics* is in being carted away to a loony bin on being caught practising these unfamiliar sounds. Find a very large, isolated field, and don't scare the animals.
* Well, studying phonetics, though you can't get away from this in beginning linguistics. Thanks, fuzzy and blue. :) | 
