Moose the Mooche was written by the bebop pioneer Charlie Parker. The composition was written shortly after Dizzy Gillespie left him to return to New York during a tour in Los Angeles. It is believed that the title refers to his drug dealer in L.A. who was nicknamed “Moose the Mooche”, as Parker was a heroine addict. This is a rhythm changes tune, meaning it is based off of the chord progression of Gershwin’s “I Got Rhythm”.
It is most commonly called in the key of Bb.