Pancho and Lefty, penned by the late great Townes Van Zandt, is a song about two young mates: one of them stronger, slicker, and the other slighter, less certain, stuck between an outlaw's life of thrills, and his own self-interested morality.
We've all been there. One day your mate puts you on a pedestal, the next day he rats you out to the feds. It's Pat Garrett and Billy the Kid, or Judas Iscariot and Jesus of Nazareth.
Van Zandt would tell you Pancho and Lefty is about "a couple of Mexican bandits". But with the author gone and the internet still here, there's an internetful of theories. It's a cautionary tale, someone says, or perhaps it's about Van Zandt himself, or even a split-personality dissociative a la Tyler Durden. At any rate, it's not an issue for Harris.
"It doesn't really matter," she told music website Pitchfork. "It's like one of those books that you want to read over and over again. The story is riveting, you feel you know the characters. It's about the human condition, it's about betrayal, it's about growing old. It's about things that we can all relate to. But it's just couched in a way that has so much beautiful imagery. It's a song that, when you sing it, your chakras are vibrating. It's just a brilliant song."
Whether it's an elegy, or a meditation on fast living versus suburban boredom, whether it has roots in truth or television, it's this one song that Harris plays at every gig - and on the phone from her home in Nashville, ahead of her Australian tour, she told Weekender she's not sick of it yet.
"Well Pancho and Lefty is unto itself ... a song unlike any other. I don't think I've ever done a set since 1976 when I did not do Pancho and Lefty," she said.
"The song still resonates with me. My audiences, and Rodney's too, we always do that song in the show.
"People who follow my music, what I've done over the years [know] there's something about that song that resonates with me. It's like a movie, it's visual. There's something so poignant about the human condition and it's just a beautiful melody and people just love it."
Country music, like the blues, is full of people doing other people's songs. In the country world, covering a song is not copying, but sharing, or paying homage. The beauty of Harris' tour is that she is playing with long-time friend Rodney Crowell, who first exposed her to Van Zandt.
"I just did it [covers albums] because I loved those songs," she says. "Of course, later on I did get into writing some of my own material. There's a wonderful songwriting community in Nashville - besides Rodney.
"Rodney introduced me to Guy and Susanna Clark, and Townes Van Zandt, and it was a whole different kind of songwriting.
"But all of us still admire the really simple, well-written country song with the great hook, which is still maybe the hardest song to write."
They were known as the "Outlaw country" singers, edgier than the mainstream of the day, outsiders. Van Zandt was never hugely popular, and it took famous singers covering Pancho and Lefty to shine a light on its genius.
Willie Nelson and Merle Haggard took it to No. 1 in 1983. By then Harris, who first recorded it in 1976, had seven years on them, and she's still going. Her versions of the song available on YouTube are a documentary study of a career, through a single song.
Naturally the way she sings it today, as a spritely 68-year-old, is different to when she first cut the track as a spritely 29-year-old. Back then her voice, always otherworldly, evoked Joan Baez, soaring and lilting, folk-melodious, dreamy and soft.
And that was exactly where Harris wanted to be. Born in the steel town of Birmingham, Alabama, she had grown up a "Marine brat". Her father served in the US Marine Corps and the family lived around various military bases. The young Emmylou, valedictorian of her high school, was a promising veterinary science student, then an actor, when she was drawn into life as a musician.
"I took a strange, circuitous route," Harris explained. "There was a folk music revival ... between Joan Baez and Bob Dylan and a whole bunch of other artists who were coming up at the time.
"Everybody told me I was musical. But I didn't really enjoy music until I got a guitar and could actually accompany myself with three chords. I still have only three chords. That's all you need!"
Of course her parents thought she could do better.
"They weren't very happy because they loved me very much and were worried for my happiness," Harris said. "As a parent, you feel like if your child is in school studying some subject, there might be a chance for some modicum of success, or to find a place with some kind of a safety net so that they can be happy, or at least have some kind of peace - you know, some kind of contentment.
"We've all suffered the slings and arrows of being alive in the world. We can't protect ourselves or our children from everything. But you want to give them as much as you can, to give them a chance."
Her folk career went slower than it might have, and her marriage to musician Tom Slocum collapsed.
By then she had a daughter, Hallie, and they moved back in with Harris' parents near Washington D.C.
"I did get very deep in the weeds with, you know, no money, and I was a single parent, and my first marriage failed and I had no skills and no future, really," she said. "They just took me in without any kind of 'we told you so' - just 'we love you, welcome home, take all the time you need, we will always be here for you'."
Only then did she really discover country music. She was making do playing in D.C. area bars when she met Gram Parsons, who became her mentor and collaborator. Harris says she had not taken country seriously before, but fell for its "subtlety, restraint, and economy of emotion", particularly the harmonies.
"That was because of Gram Parsons, when I met him, and went on the road with him and did the record," Harris said.
"And we started singing, you know, country with him and all of a sudden I realised that there was this beauty to country music that I really hadn't heard before.
"But of course when you hear Joan Baez and that incredibly ethereal voice, it's like nothing anyone had ever heard. You might have heard opera but it seemed too highbrow and you didn't understand it and there was too much going on. But you heard that disembodied voice, with her beautiful guitar playing, and the old folk ballads were so chilling.
"And then she started singing the Dylan songs. It was a combination that kind of blew the top off my mind. And I did copy her. But everybody starts by copying somebody. Hopefully, along the line you find your own style, because nobody can be Joan Baez but Joan Baez."
So Harris, recognised as one of the all-time greats of country music, with such status that she is referred to by first name alone, almost did not get into country at all. Parsons and Harris forged a musical partnership that brought together country and rock music, in the process founding what became known as alt-country.
It wouldn't be until after Parsons' sudden death that she recorded Pancho and Lefty.
Parsons, who counted among his baggage a raging heroin habit, died from an overdose of alcohol and morphine in 1973. The album he and Harris recorded, Grievous Angel, was released in 1974.
Van Zandt lasted until 53, but a life of substance abuse took its toll.
"Townes lasted a few years longer than Gram, and Gram was only 26, people forget how young he was," she says.
Harris's voice changes when remembering Parsons' early end. His death left her devastated, and the process of dealing with it has been lifelong. She has written or sung several songs about him since, most notably Boulder to Birmingham.
Post-1973, it might have been easier for Harris to fade back into obscurity, knowing her back-up plan could set her up as a well-paid and satisfied veterinarian. But that's not the kind of person we're talking about. Matter of fact, generous and hard working, Harris' motivation and self-driven confidence lead you to suspect she would make it whichever path she chose.
She describes her crossover to country in religious terms. "I was like a convert". But by 1995, when she released her 18th studio album Wrecking Ball, the genre had been gripped by what she called "virus of mediocrity". So she has explored more left-field collaborations in recent years, including with Elvis Costello, Beck, Mark Knopfler and Bob Dylan, and has won 13 Grammys.
And through it all, as her voice ages and shows more signs of the crack, or twang, that abounds in country music, two Mexican bandits have followed her on to every stage.
The reason she survived when others died young is simple. She was too smart for a heroin habit.
"I don't like to live dangerously," she said.
"I know that my friends like to drink a little wine. But I can't even do that, I'm not much of a drinker. I'm sure there's a lot of wildness and fun going on of that nature, but as you get older you find that you have fun in, I guess, subtler ways - just enjoying the fact that you're still around, and embracing each day."
■ Emmylou Harris and Rodney Crowell play the WIN Entertainment Centre on Sunday at 8pm. Tickets $109/$139.