The article presents the idea of examining the preposition auf ‘on’ from the generative perspective, in which the preposition is understood as an elementary unit of the mental lexicon (lexical array) without a syntactic category. The unit auf ‘on’ is treated as a phonological segment to which a corresponding syntactic category is assigned in the selected syntactic context. The syntactic processing system ensures the correct assignment through correct decoding from auf ‘on’ by concatenating various grammatical features with different functions and meanings. The article analyses the unit auf ‘on’ in terms of concatenation properties in the syntactic process of sentence generation in German.