The great L.A. band the Byrds can be (and are) credited with seminal innovations in folk-rock and country-rock, with singer/guitarist/lone constant member Roger (nee Jim) McGuinn’s unmistakable 12-string Rickenbacker chime sharing the spotlight with the band’s commanding vocal harmonies. The band was comprised of straight up folkies who harbored a fascination with the Beatles’ self-contained band model, and while their original compositions were excellent (if you only know the two songs that concern us here, pick up a best-of, seriously), they remain best known for an early pair of folk covers: Bob Dylan’s “Mr. Tambourine Man” and Pete Seeger’s Ecclesiastes adaptation “Turn! Turn! Turn!” Both were #1 hits, and were also the band’s only #1s.
 

 
A pair of YouTube videos endeavors to underscore the group’s impressive vocal skills by stripping away their music, and it’s really no surprise that what’s left is quite lovely. In The Dawn of Indian Music in the West, Peter Lavezzoli describes the Byrds’ harmonic process, ultimately boiling it all down to one member, guitarist/singer David Crosby:

For his part, Crosby applied his skills as a harmony singer in unconventional ways. Rather than attempting three-part harmonies like the Beatles (or five-part harmonies like the Beach Boys), the Byrds almost always employed the two-part harmony strategy of the Everly Brothers. But Crosby took the two-part approach a step further, based on his understanding of jazz and Indian modes. While McGuinn and Gene Clark sang the same notes in tandem, Crosby would move freely between a perfect fifth, flatted fifth, third, or seventh, resulting in an unusual sound that ranged from haunting to ethereal.

That kind of floored me, so I sought confirmation, and found it from McGuinn himself, who also credited the band’s harmonic gifts to Crosby in a passage from Canyon of Dreams: The Magic and the Music of Laurel Canyon:

“We sang together well,” offers Roger. “I give the credit to Crosby. He was brilliant at devising these harmony parts that were not strict third, fourth, or fifth improvisational combinations of the three. That’s what makes the Byrds’ harmonies. Most people think it’s a three-part harmony, and it’s a two-part harmony. Very seldom was there a third part on our harmonies.

 
Here’s “Mr. Tamborine Man,” helpfully synced to television footage of the original band miming the song. Chris Hillman’s hair kinda steals the show.