It is useful to think of the cardinal vowel framework like a map of an area of country that you are interested in. Obviously, if the map is to be useful to you it must cover all the area; but if it covers the whole area of interest it must inevitably go a little way beyond that and include some places that you might never want to go to. However, it is still important to know where the edges of the map are drawn. When you are familiar with these extreme vowels, you have learned a way of describing, classifying and comparing vowels. For example, we can say that the English vowel ae is not as open as cardinal vowel no.4 [a].