A microprocessor-controlled data-processing system is used to process key strokes from a musical keyboard and output a series of note events (e.g., via midi) to a tone-producing device. The microprocessor runs software which splits the keyboard into at least two zones: A root-select zone which consists of at least one octave of keys, and a strum-trigger zone. Pre-determined notes lists, or chords, are stored in an array in the system's memory. The array classifies the note lists according to (1) chord root (C, C#, D, etc.), and (2) chord type (e.g., major, minor, etc.). Different keys within the strum-trigger zone are pre-assigned to different chord types. Depression of a strum-trigger key as a root-select key is depressed causes the data-processing system to (1) select one of the note lists, and (2) output an arpeggio of the notes contained within the selected list. The note list is selected from the array based on the root note corresponding with the depressed root-select key and the chord type corresponding with the depressed strum-trigger key. Hence, each strum-trigger key performs the dual function of chord type selection and strum triggering.