I played six nights a week in a college bar just a mile from the University of Kentucky campus (63-66). When the British envasion came, the "three chorders" had some learning to do. The "4 corders", (C Am F and G guys)were forced to learn "Yesterday", "Don't Let the Sun Catch You Crying" and other tunes with pretty, but different structures.
It was an exciting time. Rock and roll had been in a rut for about 5 years. Those who learned the new structures went on to expand into jazz and other art forms incorporating the new structures.
A really significant time in pop music history.
Great mermories!
Russ