So now we have seen how four vowels differ from each other; we could show this in a simple diagram (Fig. 4). However, this diagram is rather inaccurate. Phoneticians need a very accurate way of classifying vowels, and have developed a set of vowels, arranged in a close-open, front-back diagram like Fig. 4, which are not the vowels of any particular language. These cardinal vowels are a standard reference system, and people being trained in phonetics have to learn to make them accurately and recognize them correctly. if you learn the cardinal vowels, you are not learning to make English sounds, but
you are learning about the range of vowels that the human vocal apparatus can make, and also learning a useful way of describing, classifying and comparing vowels. They are recorded at the end of Cassette 2.