The subject of this paper is a rule corpus of approx.1500 phonetic rules that models segmental variation of pronunciation in German connected speech. The phonetic rules express on a broad-phonetic level phenomena of phonetic reduction in German that occur within words and across word boundaries. The rule corpus has been designed as a component of the Munich AUtomatic Segmentation System (MAUS), which is an HMM-based system that produces the transcription of a speech signal and corresponding segment boundaries given the orthographic representation of the concerning utterance (refer to Kipp et al. [2] for details). The fact that speech is highly variable has been taken into account using the rules to complement the statistical modelling of German speech sounds and constrain the Viterbi-search. In this paper first a short introduction to the phenomenon of variability of speech and our approach of dealing with this problem in a technical application is presented. This is followed by a formal description of the syntax of the rules and the inventory of symbols that is used. Finally, I give an outline of reduction phenomena in German and how they are represented in the phonetic rules.