In this article it is argued that the selection of allomorphs is distributed over two modules, viz. Vocabulary Insertion and Phonology. This is done on the basis of a case study of vowel length alternating allomorphs in Dutch. The data show a split pattern: some empirical domains can be fully captured by phonological principles. For these cases, the phonologically most optimal allomorph will be selected. In other empirical domains, phonological principles still account for many of the attested data. Yet, one attests lexicalised exceptions as well, which are clearly phonologically non-optimal. The data echo opposing views in the literature: some proposals attempt to reduce allomorph selection to phonology, others focus on the fact that many examples are simply not phonologically optimal and suggest that allomorph selection should not be done by Phonology. I argue that the opposing nature of these two types of data is actually indicative of the way the selection of allomorphs is organised. More specifically, both Vocabulary Insertion and Phonology can determine the selection of allomorphs. Vocabulary Insertion is responsible for stored information, Phonology is responsible for phonologically optimising patterns.