Two men steeped in the rich traditions of American music, Bob Dylan and Johnny Cash, had a friendship that lasted nearly 40 years. But they only recorded together on one occasion, a two-day session that began on Feb. 17, 1969.

Dylan was at Columbia Studios in Nashville finishing the album that would become Nashville Skyline. Near the end, he was joined by Cash, whom he had first met at the 1964 Newport Folk Festival. The two were big fans of each other's work.

"I had a portable record player that I'd take along on the road," Cash wrote in Cash: The Autobiography. "And I'd put on [The] Freewheelin' [Bob Dylan] backstage, then go out and do my show, then listen again as soon as I came off. After a while at that, I wrote Bob a letter telling him how much of a fan I was. He wrote back almost immediately, saying he'd been following my music since 'I Walk the Line,' and so we began a correspondence."

"In plain terms, Johnny was and is the North Star; you could guide your ship by him -- the greatest of the greats then and now," Dylan wrote upon Cash's passing in 2003. "Truly he is what the land and country is all about, the heart and soul of it personified and what it means to be here; and he said it all in plain English. I think we can have recollections of him, but we can't define him any more than we can define a fountain of truth, light and beauty. If we want to know what it means to be mortal, we need look no further than the Man in Black. Blessed with a profound imagination, he used the gift to express all the various lost causes of the human soul."

As Jann Wenner wrote in Rolling Stone, the hope by producer Bob Johnston, who had worked with both men before (Blonde on Blonde and At Folsom Prison), was to get an entire album of duets, which would have required another session. Unfortunately, it never came to pass. Of the 15 songs they recorded in those two days, only "Girl From the North Country" -- originally found on Freewheelin' -- wound up being released, on Nashville Skyline.

But as is usually the case with Dylan's unreleased work, the tracks (embedded above) have long been readily available on bootlegs. And it's pretty easy to see why Johnston wanted another session. The two go back and forth on some Dylan songs -- "Girl," "One Too Many Mornings" -- a few Cash classics -- "Big River," "Ring of Fire" and "I Walk the Line" -- and a bunch of country and blues standards, like "That's Alright Mama," "Matchbox" and "Careless Love."

And while there's an undeniable charm to hearing this loose and unrehearsed mutual admiration society of two giants, its biggest problem is that it's loose and unrehearsed. But Dylan has trouble with some lyrics, many cues are missed and the two voices don't blend particularly well.

In fairness, it probably would have come out better if they had taken the time to work out the material before starting to record. Instead, it sounds like one of them called out a song they wanted to play and then they started to roll tape, a practice that suited the country legend just fine.

"There's nothing on earth I like better than song trading with a friend or a circle of them, except perhaps doing it with my family," Cash wrote. "As Bruce Springsteen wrote [in "Highway Patrolman," which Cash covered], 'Nothing feels better than blood on blood.'"

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