Paul Whiteman commissioned Gershwin to compose a concert piece for
solo piano and 'jazz ensemble', and he did, but instead of writing
a full score he wrote the orchestra parts in the form of a second
piano part.  On this part he wrote his instructions to Whiteman's
arranger, Ferde Grofe.  The electrifying opening glissando was
specially composed by Gershwin for Whiteman's agile clarinetist.

Gershwin was not hired to write a piece for two pianos.  Although
he had some experience arranging for show orchestras, he felt that
Grofe could do a better job than he on the detailed arrangement for
Whiteman's unique jazz ensemble.  Besides, there was precious little
time left before the concert.  He and Grofe agreed that the two-piano
format, representing the solo piano and the orchestra, was the easiest
and fastest way to communicate his ideas.

It wasn't until well after the premiere performance that Gershwin's
solo passages and improvised cadenzas were written down, and the
handwritten manuscript was then edited for publication and performance
by two pianists.  Most editions preserve Gershwin's instructions to
Grofe.  I believe the condensed version for one piano was published
much later.

Robbie Rhodes