The aim of this paper is to show the interaction between different grammatical modules in grammaticalization phenomena. Such interplay is rendered possible in particular by the underspecification present at different levels of the grammar: some lexical elements underspecified for certain features acquire functional meaning in certain syntactic positions also underspecified for their functions. It is not the syntactic position in itself which causes the acquisition of the new functional meaning, it rather depends on the interaction with the other elements in the syntactic configuration which specify each other. This quite complex process will be exemplified through the case study of the German modal particle schon. German schon is polysemous; the modal particle, the adverb and the scalar particle share a minimal common meaning, i.e. the predication of a positive state and the presupposition of a negative state on different scales. This meaning implies a contrast which is the main feature of the modal particle. All the further meaning nuances are due to a modular interplay between different factors like the verbal aspect and modality or the illocutive force of the sentence, which serve the purpose of specification of the modal meaning. As for the grammaticalization of schon, the process will be illustrated by following and then further specifying the proposal of three phases by Diewald (2002).