Emmylou Harris has shared more details about the rumored new Trio album, featuring collaborations between herself, Dolly Parton and Linda Ronstadt.

The record, which was completed using material from the women's previous recording sessions, is due for a fall release.

"We had a lot of outtakes," Harris tells Rolling Stone Country. "I didn't realize how many. That will come out in September. And it is some pretty great stuff. I go back and say, 'Damn, we sounded good. Why didn't we put that one on there?'"

The original Trio project won a Grammy in 1987, for Best Country Performance by a Duo or Group. In 1999, the threesome released Trio II, which was recorded in 1994, and the song "After the Gold Rush" won them another Grammy, for Best Country Collaboration With Vocals. This new Trio record is a labor of love for Harris, who has worked for a long time to bring it to fruition.

“We’ve been trying for years to put out the thing that we had in the can and repackage the things that we did,” Parton reveals. “Emmylou has been working at this for a long time. We thought it was coming out two to three years ago — actually even before that ..."

But the new project is bittersweet for the threesome, as Ronstadt's ongoing battle with Parkison's disease has left her unable to sing.

“I can’t get to the note,” Ronstadt says. “I can’t make any quality sound. I can’t arrange pitch. I might aim for a note and hit another one. It sounds like shouting."

Harris released an album of duets, The Traveling Kind, with Rodney Crowell earlier this year.

“It’s just me and Rodney singing,” Harris says. “I don’t know how different that will be. We’re actually working with Joe Henry to produce. We wanted to change things up just a little bit, but basically it’s just two friends singing together."

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