It is a well known property of the Germanic languages that a class of verbs exhibits systematic stem vowel alternations. In Modern German, these alternations are generally classified into two groups: ablaut and umlaut phenomena. Ablaut is traditionally recognized as a context free vocalic alternation that expresses a grammatical opposition, in Modern German the tense opposition. To our knowledge, no such morphological function has been identified for verbal umlaut. In this paper, we will investigate umlaut and argue that (i) one class of umlaut (Present Tense Umlaut) has a morphological trigger, and that (ii) PTU follows the morphological pattern of ablaut: it can be derived by the same grammatical rule. To the extent that our account is justified, it supports the assumption of morphologically contentful phonological processes.