In making the two vowels described above, it is the front part of the tongue that is raised. We could therefore describe i: and ae as comparatively front vowels. By changing the shape of the tongue we can produce vowels in which a different part of the part of the tongue is the highest point. A vowel in which the back of the tongue is the highest point is called a back vowel. If you make the vowel in the word 'calm', which we write phonetically as ...:, you can see that the black of the tongue is raised. Compare this with ae in front of a mirror ; ae is a front vowel and ..: is a back vowel. The vowel in 'too' (u:) is also a comparatively back vowel, but compared with ...: it is close.