Listening to Pink Floyd's song "Shine On You Crazy Diamond" all parts again, and I heard a vewwy interesting chord.
Specifically, during the saxophone solo, and the slide guitar solo later on we have an Eb7 chord, which moves to a D chord, which then moves to Gm. Now, the song's firmly in the key of G minor, so the two most important chords in G minor are Gm, that's the Tonic, duh, and D or D7, that's the Dominant in G minor.
Now, when I hear them go to Eb in the bass, and the chord I hear over that is an Eb7 chord, that creates a very interesting chord, and has some interesting harmony.
So, the notes in an Eb7 chord are Eb, G, Bb, and Db.
Now, as those notes relate to G minor, our key, those notes are:
Eb = minor sixth.
G = root/Tonic.
Bb = minor third.
So the only note that's chromatic in that is Db, which would be a tritone.
So you have Db resolving up to D, and you have Eb resolving down to D, forming the basic foundation of a D maj chord or D7 chord, which is the V chord, which points us back to Gm, our home chord.
So that Db is really a #4 and that Eb is really a b6, and both resolve towards D at the same time.
That makes that Eb7 chord in the song an augmented sixth chord, specifically, a German augmented sixth chord, because the minor third, Bb, is in the chord.
Such chords are heard all over the place in classical music, and rarely heard in popular music, especially rock.