Billy Strayhorn's composition "Take the 'A' Train" was the Duke Ellington orchestra's signature tune. The piece's introduction is instantly recognizable and prepares the listener for the rest of the classic composition.
Here's a transcription of the famous 4-bar intro (click to enlarge):
It is comprised of just two chords. The first chord is a second-inversion C major chord, and the second chord can be thought of as a first-inversion D9#11 (though it lacks the root). Using an enharmonic spelling, it could also be thought of as a third-inversion Ab7#5, but the D9 interpretation seems more fitting, if slightly incorrect, considering the D7#11 chords found later in the head.
These two chords are covered by the left hand, and the right hand plays melodically above. The right hand covers the span of a minor sixth, connecting E to the G# below with a glissando. The minor sixth is notable in that it is just a half step smaller than a major sixth, which is found in the first few notes of the tune's melody (G up to E). The introduction's minor sixth, in a sense, creates tension (as does the whole-tone color produced by the #11 chord), which is subsequently resolved on bar 1 of the head.
Have a listen to Duke Ellington and his orchestra playing 'A' Train (begins at 0:12):