The same but different again

Most of the features which work well in classifying and describing consonants are entirely inappropriate for vowels, while vowels vary in dimensions (such as tongue height) which are not relevant for consonants. However, when we turn to the criteria for establishing phonemes, it turns out that vowels and consonants behave very similarly indeed. Below therefore fulfil a dual role of providing more information about vowels, while allowing some revision of notions like complementary distribution, allophonic rules, free variation, neutralization and phonetic similarity, which were first introduced mainly in connection with consonants.