Dimensions of vowel description

The three vowels we have just described form three corners of the IPA’s vowel quadrilateral. This represents, schematically, the vowel space: sounds articulated at the edge of or inside the box are vowels. The vowel space uses three dimensions for describing vowels: vowel height, frontness and backness, and lip posture (or rounding). If the tongue is raised any higher than the sounds along the [i]–[u] axis, then friction is generated, and so a fricative (i.e. a consonant sound) is produced. If the tongue is lowered or backed beyond the vowel [ɑ], then friction is also produced. The cardinal vowels take pre-determined positions in the quadrilateral; other vowels are fitted in the spaces in between.

Vowel height is represented on the vertical dimension: from close through close-mid, and open-mid to open, along a continuum. The horizontal dimension of the chart (front – central – back) represents the second aspect of vowel description: vowel frontness/backness. The points between the extremes are chosen because they are useful reference points. In theory there is an infinite number of points between the extremes on each dimension.

The third aspect of vowel description is lip posture. The lips can be held in a large number of postures. Here are a few: spread and close (as if smiling), spread and protruded (as if to make a rectangular box between the lips and teeth), compressed and protruded (as if to make a polite kiss on someone’s cheek), and open and rounded (as if to make a big O-shape). The IPA represents lip posture implicitly in symbols, with diacritics for deviations from what is implied.

There are two sets of cardinal vowels: the eight primary cardinal vowels [i e ε a ɑ ɔ o u] and the secondary cardinal vowels  . The primary cardinal vowels are, as the name suggests, the more important ones, because they represent the commonest types of vowel across the world’s languages. The secondary cardinal vowels are less common. They are the same as the primary cardinal vowels with respect to tongue posture, but they have reversed lip postures: for example, [y] has the same tongue position as [i], but the same lip posture as [u]; [ɯ] has the same tongue position as [u], but the same lip posture as [i].