The idea of segments
When you speak, there are several different things going on in your mouth at the same time. For example, when you say the sequence [un], at least the following are happening:
your tongue tip raises from a relatively neutral position till it touches your alveolar ridge and cuts off airflow through the mouth
your tongue body starts in a high position and drops to a relatively neutral position
your lips start out rounded and become less rounded
your soft palate (velum) starts out raised, blocking off airflow through the nose, but becomes lower and lower, allowing more and more air out through the nose
your vocal cords are vibrating
The parts of the vocal tract are (mostly) independent of each other. Several parts are changing, but they aren`t necessarily changing all at the same time. For example, for most English speakers saying [un], the soft palate lowers and lets air out through the nose long before the tongue tip raises and blocks off airflow through the mouth. The whole set of movements by four parts during the word spoon might look like: (1)








It certainly doesn`t look like:
(2)






On a practical level, it`s almost impossible to start work using the first kind of picture, even if it is more realistic. To begin studying phonetics, we have to idealize the real world. We have to act as if speech events look like the second picture rather than the first. We can divide the idealized picture up into segments which have well defined properties. Figure (2) has four segments:
a segment with unrounded lips, the tongue tip near but not touching the alveolar ridge, neutral tongue body; raised soft palate, and spread vocal cords -- we can write this as [s];
a segment with unrounded lips, neutral tongue tip, neutral tongue body, raised soft palate, and spread vocal cords -- we can write this as [p];
a segment with rounded lips, neutral tongue tip, high tongue body, raised soft palate, and vibrating vocal cords -- we can write this as [u]; and
a segment with unrounded lips, the tongue tip against the alveolar ridge, neutral tongue body, lowered soft palate, and vibrating vocal cords -- we can write this as [n].


When phoneticians and phonologists talk about a sound, they are usually referring to one of these idealized segments.
Ultimately, a theory of phonetics and phonology has to explain realistic pictures like (1) rather than simplified pictures like (2). But for many practical purposes, it`s good enough if we use the idealization that any stream of speech can be broken up into a series of segments and each segment given a phonetic symbol.



Consonant parameters
What makes one consonant different from another?
Consonants involve constrictions, or gestures that narrow the vocal tract at a particular point. Most consonants use only one constriction, but some have more than one. In order to fully describe a consonant sound, we need to describe each constriction as well as some other properties of the vocal tract.
Parameters of a constriction gesture
active articulator: What articulator moves to make the constriction?
passive articulator: What articulator does the active articulator touch or approach?
degree of constriction: How close do the active and passive articulators get to each other?
(laterality)
These should be specified for every constriction involved in a consonant. The following properties don`t apply to any particular constriction, but to the consonant as a whole:
Other parameters
state of the glottis: What are the vocal cords doing?
nasality: Is air escaping through the nose?
airstream mechanism: How is the flow of air being produced?
Answering each of these questions about the state of the vocal tract is enough to uniquely identify any consonant. If two consonants are different, they must differ in their answers for at least one of these questions.
Together, the active and passive articulator are often referred to as the place of articulation (POA). The other parameters are often lumped together and referred to as manner of articulation.


Place of articulation
The active articulator usually moves in order to make the constriction. The passive articulator usually just sits there and gets approached.
A sound`s place of articulation is usually named by using the Latin ajective for the active articulator (ending with an "o") followed by the Latin adjective for the passive articulator. For example, a sound where the tongue tip (the "apex") approaches or touches the upper teeth is called an "apico-dental". Most of the common combinations of active and passive articulator have abbreviated names (usually leaving out the active half).
These are the abbreviated names for the places of articulation used in English:
bilabial
The articulators are the two lips. (We could say that the lower lip is the active articulator and the upper lip the passive articulator, though the upper lip usually moves too, at least a little.) English bilabial sounds