Rearrangement
What makes the original recording(s) so interesting is the fact that Ellington has taken the traditional front-line of trumpet, trombone and clarinet, and turned them "upside down." At the time of these first three recordings in 1930, the usual voicing of the horns would be clarinet at the top (highest pitch), trumpet in the middle, and the trombone at the bottom (lowest pitch). In "Mood Indigo," Ellington voices the trombone right at the top of the instrument's register, and the clarinet at the very lowest. This was unheard of at the time, and also created (in the studio) a so-called "mike-tone"—an effect generated by the overtones of the clarinet and trombone (which was tightly muted as well). The "mike-tone" gives the audio-illusion of the presence of a fourth "voice," or instrument.
Ellington was to re-employ this effect in "(In My) Solitude" (1932), "Dusk" (1940) and many other pieces throughout his career. The Ellington band performed and recorded the song continuously throughout its 50 years under Duke, both in its original form and as a vehicle for individual soloists. It remains a staple of the Duke Ellington Orchestra.
Read more about this topic: Mood Indigo