The paper presents a corpus-based information-structural analysis of the German discourse particles aber and auch in the so-called post-initial position, i.e. between the pre-field constituent and the finite verb in main clauses. Based on corpus data, I argue that aber and auch in this position mark the border between the focus and the background of the sentence that hosts them. By doing this, they essentially contribute to identifying relevant alternatives involved in focus interpretation and discourse interpretation in general. In this way, the placement of particles in postinitial position facilitates interpretation by providing processing instructions to the reader/hearer and by ruling out ambiguities. The analysis argues thus against existing accounts of the discourse function of post-initial particles in terms of indicating association with topic and topic shift.