Melody note data and chord data are inputted to an automatic arrangement apparatus. When it is detected that a melody note is a nonharmonic note, the melody note is converted into a harmonic note, and an additional note (e.g., a countermelody note) is generated based on the converted harmonic note. An electronic musical instrument comprises such an automatic arrangement apparatus. Melody note data and chord data are inputted to the instrument according to a performance of a player. When it is detected that a melody note is a nonharmonic note, the melody note is converted into a harmonic note, an additional note is determined based on the converted harmonic note, and the additional note is produced as a tone in real time simultaneously with a performance tone.
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