Charles Ives, Three Quarter-Tone Pieces, Mvt. II, Allegro, mm. 3-6 (original notation)
Scored for two pianos, one tuned a quarter-tone sharp. Listen for the descending bass line at the beginning of the excerpt—you can see how the line is distributed between the two instruments. This system of notation is convenient for pianists because they can play from the score as they normally would and the the tuning of the two pianos takes care of the quarter tones; the piano with the altered tuning is, in effect, a transposing instrument. The disadvantage of this scheme is that the notation makes it harder to see the voice leading.