It refers to the actual relationship between articulators and thus the way in which the air passes through certain parts of the vocal tract.

It refers to where in the vocal tract there is approximation, narrowing or the obstruction of air.

(a) the articulators may close off the oral tract for an instant or a relatively long period; (b) they may narrow the space considerably; or (C) they may simply modify the shape of the tract by approaching each other.

It refers to sound segments produced when the airstream that comes from the lungs meets no obstruction of any kind in the throat, the nose or the mouth, so no turbulence of a total stopping of the air can be perceived.

Vowels are differentiated by a number of factors:

(1) the height of the tongue raising---high, middle or low,

(2) the position of the highest part of the tongue- --front, central or back,

(3) the length of the sound--- tense, lax,

(4) the shape of the lips--- rounded, unrounded. When we describe the vowels, the first four factors are sufficient, for example, [倒e] central lax unrounded vowel; [i:] high front tense unrounded vowel; [u] high back lax rounded vowel. They are the nucleus of the syllable.

The most famous“cardinal vowels” system of all was put forward by Daniel Jones.

All cardinal vowels are monophthongs and their quality does not change during their production. They include eight“primary" cardinal vowels and eight“secondary" cardinal vowels.

The cardinal vowels are a set of vowel qualities arbitrarily defined, fixed and unchanging, intended to provide a frame of reference for the description of the actual vowels of existing languages.