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Try to read aloud the bubbled-sentence in the cartoon "t's tm fr m t kp slns". Can you make yourself heard? No, because consonants don't carry much loudness.
Now look at the following list of Chinese words. What are the similarities and differences in their pronunciations?
拉（la）、乐（le）、历（li）、录（lu）、来（lai）、老（lao）、楼（lou）、累（lei）
The pronunciations of all of the above words begin with the same consonant [l], and what distinguish them from one another are the vowels.
So vowels are important in our speaking. They not only distinguish one word from another, but also make our speaking audible. They carry the voice, pitch and loudness in our speaking.
Consonants and vowels differ from each other chiefly in the manners of articulation. In the articulation of consonants, the air may come across different modifications in the vocal tract. As for the production of vowels, the air comes across hardly any obstruction and it flows along the vocal tract fairly freely. However, the distinction between consonants and vowels is not absolute. There are some marginal sounds, such as the approximants [r, l, w, j]. They share some of the vowel qualities—in the articulating, the air comes across hardly any obstruction. So sometimes they are called semi-vowels.
In describing the consonants, we make use of the factors such as the places of articulation or manners of articulation. We try to figure out how the air is modified and where the modification takes place. But the production of vowels doesn't involve any articulators touching or even coming close to each other. What then are the phonetic properties that make each vowel sound a distinguishable one?
If you watch x-ray movies of someone articulating different vowel sounds, you will find he is changing all the time the position of his tongue and the shape of the lips. What differentiates the articulation of [i] as in "it", [u] as in "who" and [ɑ] as in "Ah" is how the tongue and the lips shape the oral cavity. So to describe the vowels, we turn to new factors:
(1) the highest point of the tongue along the vertical axis—HIGH, MID, or LOW;
(2) the highest point of the tongue along the horizontal axis—FRONT, CENTRAL or BACK;
(3) the degree of LIP-ROUNDING—rounded or unrounded.
For example, to articulate [i], the tongue is raised very high in the mouth, and the highest point is the front of the tongue. As for [ɑ], the front of the tongue is lowered and the highest point is the back of the tongue. In articulating both of the two sounds, we need to spread our lips. Therefore, the [i] sound is "high, front, unrounded", and [ɑ] is "low, back, unrounded". Fig. 2-4 shows the tongue position variation in the articulation of [i] and [ɑ].
Fig. 2-4 Tongue position variation in the articulation of vowels
The words we use to describe the properties of vowels, such as "high" and "low", or "back" and "front", are not so definite in meaning. Because in order to define a certain position as "high" or "low", we should make some comparison. For example, we'd better say "[x] is a high vowel because it is produced with the tongue higher in position than [y]". That means we need some reference points to describe vowels. We can start by defining some "standard" sounds. Then we can describe other vowels by making a comparison between the reference sounds and the sounds we want to define. Such reference points are defined in the CARDINAL VOWEL SYSTEM.
We may represent the Cardinal Vowel System by a Cardinal Vowel Chart. (See Fig. 2-5) The chart is an attempt to simulate the shape of our mouth—The "high" position is closer to the palate than the "low" position; the "front" position is nearer to the lips than the "back" position. The eight numbered points on the chart are the eight cardinal points intended to serve as standard reference points. They are called the eight PRIMARY CARDIVAL VOWELs.
Fig. 2-5 Cardinal Vowel Chart
Cardinal 1 is the vowel produced when we reach the tongue as high and far forward towards the hard palate as possible, making sure that it will not cause any audible friction. Cardinal 5 is the vowel produced when we draw the tongue as low and retracted as possible. These two sounds are the easiest to feel, and they serve as the starting point of the system. From the position of cardinal 1, we lower our tongue little by little but still keep it front, we will have Cardinal 2, 3 and 4. Starting from cardinal 5, we raise the tongue bit by bit in even space, and cardinal 6, 7 and 8 are thus produced. We can put other vowels on this chart, and they can be described according to their relation with the cardinal points.
The vowels we have discussed so far are all simple vowels called MONOPHTHONGs. However, there are vowels which may be described as a sequence of two sounds, or the glide from one vowel position to another. Such sounds, i. e. DIPHTHONGs, are still treated as single sound segments. There are different types of vowel glides. If we start from a low vowel [a] and glide towards a higher [i] position, we are producing a rising diphthong [ai]. Similarly, by gliding from the front [i] to the central [ə] position, we may articulate a centering diphthong [iə].