One of the many provocative (or brilliant) (or crazy) assertions Ian MacDonald makes is his Revolution in the Head concerns the relationship between the personnel of a band and the band’s songwriting. MacDonald’s entry on “Helter Skelter” begins thus: 

The ‘heavy metal’ idiom of the Seventies originated in the mid-Sixties switch from the low-volume standard pop four-piece to the vastly amplified rock ‘power trio’, a format change in which the redundant rhythm guitarist was replaced by turning up the bass, close-miking the drums, and adding a range of signal-distortion effects to the lead guitar. Led by groups like The Who, Cream, and The Jimi Hendrix Experience, this move was, to some extent, an inevitable consequence of bigger and better amps and speakers designed for larger and more remunerative venues. Yet the loss of the craft of the rhythm guitarist was soon felt in a degradation of texture and a decline in overall musical subtlety. Rhythm guitarists were usually songwriters, and the variety of articulation and accenting techniques they used also shaped their compositions. The average power trio, lacking such a musical brain, was in effect an excuse to replace songs with riffs and discard nuance for noise. 

Is this claim true? Even though I’ve thought about it a lot since I’ve first read it, I’m still not sure. But here’s an interesting data point. In his biography of John Lennon, Philip Norman quotes George Martin on how different John and Paul were in their songwriting practices:

“Paul would think of a tune and then think 'What words can I put to it?' John tended to develop his melodies as the thing went along. Generally he built up a song on a structure of chords which he would ramble and find on his guitar until he had an interesting sequence. After that, the words were more important than anything else. They used to come out sometimes as a monotone, just one note punctuated by the rhythm of the words. He never set out to write a melody and put lyrics to it. He always thought of the structure, the harmonic content and the lyrics first, and the melody would then come out of that.” 

And it makes sense that a rhythm guitarist, who spends most of his time playing chords — and maybe also looking for some new or different chords to play — might be especially attentive to “the structure, the harmonic content.” And that in turn might lead to songs with unexpected chord progressions, which John Lennon’s songs have plenty of. 

As I’ve noted before, guitarists who play in standard tuning are helped in this search by certain elementary principles of physics: among the common chords (“cowboy chords”) the two easiest to make are D major and A minor. And once you’ve gripped one of those it’s the easiest thing in the world to slide up a couple of frets to see what that sounds like. And then maybe a couple of frets more. You can find some weird and wonderful harmonies that way. And new chords and new harmonies are what the rhythm guitarist needs — unlike the lead guitarist, who has other business to attend to, business that doesn’t often result in the discovery of an unexpected melody.