Emmylou Harris and Rodney Crowell had been friends for almost 40 years by the time they got around to making an album together. “Old Yellow Moon,” out today, is a loose-limbed mix of honky-tonk, old-school country and vintage, Topanga Canyon country-rock that draws on their shared past.

Produced by Harris’s ex-husband Brian Ahern, who first brought the two together in the early ’70s, “Moon” is a sepia-toned collection of mostly plaintive covers (of Kris Kristofferson’s addiction lament “Chase the Feeling,” of the Waylon Jennings-popularized “Dreaming My Dreams,” now a duet) and Crowell originals revisited (such as “Here We Are,” which Harris once recorded with George Jones, and an updated version of their first collaboration, “Bluebird Wine”). Harris’s voice retains almost all of its silvery loveliness, Crowell’s writing is as tight and sharp as piano wire and “Moon” just generally sounds as if it was recorded by friends, around a kitchen table, which is partly what happened.

In semi-separate phone interviews (Crowell, 62, did his alone; Harris, 65, did hers sitting next to Crowell, and occasionally consulting him about half-remembered details, in the Nashville airport), the two old friends talked about their new album and traced the origins of their friendship from its earliest days to today.

In 1974, after the death of her mentor Gram Parsons, Harris, a young single mother, was living with her parents in the Maryland suburbs. Crowell was a struggling songwriter.

Emmylou Harris: I was working the [D.C.] clubs: the Childe Harold, Mr. Henry or Oh Henry’s, whatever, the Assembly, the Red Fox Inn. I was pretty much doing six nights a week. I heard [Rodney] up in Toronto when I was working on material with Brian Ahern. I heard his demo and got very excited, and [Ahern] arranged for us to meet.

Rodney Crowell and Emmylou Harris. (David McClister)

Rodney Crowell: She was playing at the Childe Harold, this old folk club. I went to see her, and then we stayed up all night, sitting on the floor. We started a conversation about songs and music, the Louvin Brothers, Townes Van Zandt, Guy Clark. And that very conversation that night is a conversation that we’ve sustained over the years, and was really what led to [this album], truthfully.

Harris’s major label debut, “Pieces of the Sky,” was released in early 1975; “Bluebird Wine,” written by Crowell, was the lead track. Crowell and Harris re-recorded the song, with new lyrics by Crowell, for “Old Yellow Moon.”

Crowell: When [“Pieces of the Sky”] came out, I was in California with her. We were in a boutique in Westwood and “Bluebird Wine” came on the radio, and we were like those kids in that Tom Hanks movie “That Thing You Do,” we were jumping up and down. We go up to the cashier, and [Emmylou said], “Listen, do you hear that? That’s me singing.” And I said, “Yeah, and that’s my song.” And the cashier sort of lazily said, “Oh, yeah, sure.” We were like, “No, really. It is!”

Harris: Rodney had never recorded it himself, and it seemed like [a new version would] give everyone a chance to hear what I heard when I heard Rodney sing that song. And I suppose there was a certain sentimentality for me, because it is kind of how we got together.

The following year, Crowell and Harris met up again in Texas. He soon joined her vaunted backing group, the Hot Band.

Crowell: She came to Austin, where I was at the time, and played the Armadillo World Headquarters with a band she had from D.C. I sat in with her, and after the gig she said, “Hey, I’m going to L.A. tomorrow. I’ve got a spare ticket. Want to come?” Emmy doesn’t remember having a spare ticket, but there’s no way I could have afforded to go on my own.

Harris: He didn’t have any place to land in particular . . . I needed someone to play rhythm guitar and sing duets with, and at that point we’d been sitting around the house [in L.A.], singing country songs. It just seemed like a natural fit, that of course he’d be in the Hot Band.

Crowell: They were gonna hire these big, high-priced band guys that had played with Elvis to go on the road with her, and I was a friend and confidant and source of songs . . . I landed my role in that band on the strength of the songs, and as a source of friendship for Emmy.

Crowell and Harris remained close throughout the next few decades, as their careers progressed on parallel tracks. As Harris’s fame grew, Crowell became a country star in his own right, and was one-half of a Nashville power couple with his then-wife, Rosanne Cash. Many of the songs on “Old Yellow Moon” are tracks Harris and Crowell had always wanted to record together.

Crowell: Johnny Cash did “Bull Rider” all those years ago, and he did make it iconic, and it was beautiful. Then two or three years ago Norah Jones recorded it on one of her records, and when I heard it . . . it took me a moment to recognize it was my song. I played it over and over and over again, because I loved it. Something about her version of it gave it back to me; that’s the only reason I brought it up for the sessions with Emmy. She always loved that song.

Harris: To me, the song that exemplifies this record and our friendship is the title track. It could be about a marriage; it could be about a brother and sister; it could be about friends. It’s just about a long, enduring affection for someone.

Harris and Crowell would sometimes go years without seeing each other, and easily pick up their friendship where they left off. Crowell says he can talk to Harris about things he talks to his male friends about, such as baseball. She figures it helps that they never dated.

Crowell: I don’t ever remember us having an argument.

Harris: Oh no, we’ve never had a fight. If we had disagreements, it’s about a song or an idea, and we talk it through.

Crowell: I think I lighten Emmy up, and she lightens me up. It’s always been that way.

Harris: He definitely lightens me up. I hope I lighten him up. He’s one of those people — his friendship is so important. There’s some people you just delight in, and Rodney’s one of those people for me.