liquid, typically water. It is thus related to the pagophone, an instrument that uses ice, and the calliope, an instrument that uses steam. 12-jet diatonic Many diatonic hydraulophones are built with 12 water jets, one for each of the instrument's 12 notes. The standard compass starts on A, extending up an octave and a half to E. The standard A to E range, in which it is possible to play with polyphonic embouchure on any or all diatonic notes at the same time, is shown on the left side of the diagram. When playing only monophonically, some additional range is possible on certain hydraulophones, indicated here by small cue notes at the end-points. Left, the extended notes come from closing key change valves or flexing key change levers, for sharpener, and flattener. To play a low G, one must be playing in C minor (with A) and close the flattener valve simultaneously. When playing on the high E jet, closing the sharpener valve produces an F. 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". Finally, on the right, the additional extended range comes from the two octave-change valves (all notes can be shifted as many as two octaves down, or one octave up). Markings Because the water-spray from hydraulophones obscures vision (or because hydraulophones are played underwater where visibility is poor), finger holes are sometimes encoded in Braille. Braille has the added advantage that the one-to-one correspondence between letters and numbers is the same as the standard A-minor hydraulophone, i.e. jet 1 is A, jet 2 is B, jet 3 is C, etc.. The skill (intricate sense of tactility) needed to play a hydraulophone well is also similar to the skill needed to read Braille. (12 sets of dots typically made from