"The Rhapsody was performed by Whiteman's band, with an added section of string players, and George Gershwin on piano. Gershwin decided to keep his options open as to when Whiteman would bring in the orchestra and he did not write down one of the pages for solo piano, with only the words "Wait for nod" scrawled by Grofé on the band score. Gershwin improvised some of what he was playing, and he did not write out the piano part until after the performance, so it is unknown exactly how the original Rhapsody sounded.
The opening clarinet glissando came into being during rehearsal when; "... as a joke on Gershwin, [Ross] Gorman (Whiteman's virtuoso clarinettist) played the opening measure with a noticeable glissando, adding what he considered a humorous touch to the passage. Reacting favourably to Gorman's whimsy, Gershwin asked him to perform the opening measure that way at the concert and to add as much of a 'wail' as possible."