With change-valves, the diatonic hydraulophone is polyphonic in the same sense as a so-called "chromatic harmonica" – you can play chords and move all members of a chord down one semitone or up one semitone together, but the function of the valves is usually not separated to work on a per-note basis, so for example, you can play an A-minor chord, and flex the entire chord down to A-flat minor, but you can't easily play an A major chord without the use of polyphonic embouchure to bend only the middle note to a C (which requires more skill than the average hydraulist has). Thus the "diatonic" hydraulophone is called "diatonic" conservatively to "under promise and over-deliver".