Microtones have long been a structural feature of Asian music. The use of microtones in Western music, although far from new, has been -- aside from traditional, empirical performance practice involving microtonal adjustment of intervals for expressive purposes -- far less extensive.
Around the beginning of the 20th century, when Western music began reaching its chromatic saturation, composers began using the microtonal system as a resource. Between 1903 and 1914, Charles Ives wrote a "Quarter-tone Chorale for Strings". He used quarter-tones in at least two other works, "Quarter-tone Pieces for Two Pianos" and his Fourth Symphony. Harry Partch experimented with dividing the octave into 43 unequal steps. More recently, composers of electronic music have incorporated microtonal processes quite extensively.
A few acoustic instruments have been built for the purpose of performing microtonal music. In 1924, in Germany, a quarter-tone piano was built with two manuals, one being tuned a quarter step higher than the other. Later, in the United States, Hans Barth built a similar instrument. There also exists an organ designed to play 31-note (within the octave) music.