This is one of the greatest Gershwin tunes ever written, instantly recognizable from the opening chords. He wrote this tune originally as an up-tempo dance number. It was his brother Ira who said, George, play that again, slow.
This is George at the piano in 1926 playing it the way he wrote it originally. The middle eight bars are incredibly beautiful, even though the faster tempo doesnt do them justice. In a crossover, he even moves the theme to the bass while the left hand plays block chords in the treble.