The first six bars consist of a series of highly dissonant chords built over concert B-flat and concert A-flat.  It is interesting that, while no particular tonal centre seems to be implied from the point of view of traditional harmony, the movement on either side of concert A (which is later defined as the tonic) is typical of many jazz progressions at a final cadence.  The overall effect, however, is primarily percussive. All this changes when we get to section 1, which is built almost entirely on an Em7b5 chord.  The point at which this chord is introduced (first measure of section 1) is therefore very important and should be emphasized.  The tension is heightened by the introduction of the ninth in the brass chord two bars before section two (bar 12 and 13).  The resolution of this chord on the first beat of the first bar in section two establishes the tonic as A.