Pink Floyd’s best album came out 40 years ago, one of the most successful ‘concept’ albums in rock history. In addition to the other aspects which tie the songs here together is a musical relation also found in many other classic rock pieces.
We hear it in this album right away, in “Breathe In the Air”: two chords, one minor and one major, which form a repeating cycle, over which we hear both guitar licks and the main vocals. This two-chord harmonic loop returns for the famous improvised vocal which closes Side 1, although this time in a higher key. Then it comes back *again* for the third song on side 2, in yet again a different key.
It’s a great sequence for open-ended improvisatory jams, and Pink Floyd makes the most of it. Having this musical shape distributed across these songs helps tie the whole album together musically, strengthening the ‘concept’ and giving the impression that these songs belong, together in this order, as one large work in multiple sections.
The same chord sequence is used in another monument of rock, by the way, in “Stairway to Heaven”, as part of the brief musical interlude between verses. It’s a very effective harmonic progression.
For the purposes of “Dark Side of the Moon”, here’s the thing: these two chords never give you the actual key of a song. Ordinarily, they would not appear together. For “Breathe In the Air”, the chords are E minor and A major. If you’re in the KEY of E minor, you don’t have an A major chord, it would be A minor (the subdominant or iv chord, for those keeping score). If you’re in A major, then you have E Major, an even more important chord (the dominant, or V chord).
So this loop leaves you hanging. Conceivably you could interpret these as the ii chord and V chord of a key–but sooner or later you have to reach the I chord (the tonic). Lots of songs do just that, ii-V-I, very common.
Pink Floyd never does. This song finally rotates out of the loop through a sequence of chords that would ALL be in other keys, and **sort of** claims a very ambiguous E minor by the end, just by returning to this two-chord loop.
There is ONE place where Pink Floyd does touch down, so to speak, harmonically, exactly 3:06 into “The Great Gig In the Sky” on the end of side 1. We’ve been vamping the two chords, G min and C, for a long time, and finally it DOES go through as ii-V-I, which in this key is G min, C, to F, right at 17:41 into Side 1. And then it takes right off into outer space again…
It’s an amazing moment. Harmonically Pink Floyd has held the listener suspended all this time–the payoff at this single moment owes its impact to all that floating back and forth which went on for such long periods before.