It has been demonstrated that metaphony is triggered by underlying final /-i/ and /-u/. The important feature that unifies the two posttonic vowels, which are responsible for the tonic vowel alternations, is [+ high[. The spreading of this feature to the tonic vowel accounts for metaphony. Vocalisms defined without the feature [high], such as those of particle phonology and Goldsmith's model, fail to mirror the metaphony process.