We are developing systems which play music on synthesized plucked strings instruments idiomatically. The present paper introduces a system focusing on the acoustic guitar. The system is programmed with rules embodying the architecture of the guitar and the respective biomechanical constraints of a performer. These rules define the types of musical gestures that are played by the system: only those gestures that are idiomatically compatible with the guitar are performed. The system also features rules for arranging (and re-arranging) musical materials in order to make them idiomatically playable. These include rules for shaping chords and algorithms for generating realistic fingering.