In many languages there are words that behave like lexical verbs and on the one hand and like functional categories expressing distinctions of tense, aspect, modality, etc. on the other. The grammatical status of such words is frequently controversial; while some authors treat them as belonging to one and the same grammatical category, others assign them to different categories. The present paper is concerned with such a case of "doublets": Looking in more detail at the German item "drohen" "to threaten", it attempts to offer an account of the differences in the lexical and functional structure of this item by using grammaticalization theory as a framework. As the data presented in the paper suggest, this model in fact accounts for most though not for all of the properties characterizing the structure of the item. It is argued that in addition to synchronic analysis, a diachronic and comparative perspective is required in order to arrive at a more comprehensive understanding of the interface between lexical and functional structures. (Contains 8 tables.)