Metaphony (German Umlaut) is that kind of vowel harmony (a phonological process) which converts back vowels into front vowels. In the Germanic languages in which it is well documented, it is conditioned by a front vowel in the syllable following the affected syllable. Moreover, metaphony is secondarily morphologized in these languages as the conditioning environment got lost and the morphological features originally born by it then shifted onto the root affected by metaphony. Metaphony thereby becomes a morphological process. For instance, the metaphonic vowel alternation in English tooth - teeth signals different values of the category ‘number’.