I build two types of double flutes: The first, as seen above, is a drone flute.  It has two flutes of the same length, one of which has no fingerholes.  The flute is fingered exactly like a single flute, except that you also blow simultaneously into both flutes, giving you a drone note alongside the melody you play.  By blowing only into the fingered side, you can play single notes without the drone as well.
The second, as seen below, is a true double flute, with two flutes of different sizes and keys played simultaneously.  With one hand on each flute, you play two notes at the same time, with a 5-note range on each side.  Effectively, a soloist can play two flutes at once, albeit with a slightly smaller range on each (you’ve only got one hand per flute.)  Still, across the two flutes, you can achieve a range equal to a standard flute.  You can also play either side independently, for a single melody line.  The flutes are in keys set a fifth apart, such that playing identical fingerings on both sides will give parallel fifths, but by changing notes you can play any interval from a minor second to a full octave. I usually make double flutes in the middle and lower registers (A and below, down to D), to make best use of the resonant low tones, but have also made higher-register flutes.  For more information on scales, keys, and woods used, go back to the main flute page.