In 1975, Pink Floyd released what I consider to be their best album, Wish You Were Here. The album consists of five tracks:
Despite its seven-and-a-half-minute length, 'Welcome to the Machine' employs only four chords: E minor, C major, A minor, and A major. Of those four, the A major might be the most curious because it is heard only once (during the second verse, at 4:18), where it substitutes for the A minor chord heard during the initial verse. The obvious question, then, is why? Why does the second verse use an A major chord where the first verse uses an A minor? What is the point of making that substitution? Before we can answer that question, we first need to progress to the next track.
Like its immediate predecessor, 'Have A Cigar' also uses only four chords: E minor, C major, D major, and G major. Notice that two of these four chords (E minor and C major) were used in 'Machine', while the other two (D major and G major) were not.
So we have seen how 'Machine' uses the chords E minor, C major, A minor, and A major, and how 'Cigar' uses E minor, C major, D major, and G major. Now, given the title of this blog, can you make an educated guess regarding what chords are used in the title track? I'll give you a hint: There are six of them.
Those six chords are E minor, C major, A minor, A major, D major, and G major. They should be very familiar since they are the combined harmonies from the two previous tracks - no more, no less. This is what I mean by the term "chordal accretion".
Here's the same concept using color-coded mathematical symbols:
The chords of 'Welcome to the Machine' (E minor, C major, A minor, A major)
The chords of 'Have A Cigar' (E minor, C major, D major, G major)
The chords of 'Wish You Were Here' (E minor, C major, A minor, A major, D major, G major)
So, getting back to that curious A major chord in 'Welcome to the Machine', the reason it's an A major in the second verse instead of A minor is to help set up this chordal accretion process, which culminates in the focal point of the entire album, the title track.
Aaron Krerowicz, pop music scholar
An informal but highly analytic study of popular music.