When you listen to the intimate sound of the Indigo Girls music, somehow it doesn't seem surprising that Emily Saliers and Amy Ray have known each other for a long time.

The singers, both 45, attended Laurel Ridge Elementary School just outside of Decatur, Ga. They became friends during high school, and went on to forge a long-lasting bond that's rare in an era of solo vanity projects.

"She does what I can't do, and I can do what she can't do," Saliers said of her creative partnership with Ray in a Tribune interview.

The women began performing together at Shamrock High School. After graduation, when Sailers left town to attend New Orleans' Tulane University, Ray frequently visited and the pair began busking together in the French Quarter. Saliers and Ray both ended up transferring to Georgia's Emory University, where they changed their performing name to the Indigo Girls -- just because they thought the name sounded cool.

The duo got an early career boost in the late 1980s when female songwriters such as Tracy Chapman, Natalie Merchant and Suzanne Vega became the flavor of the moment. The Indigo Girls were also part of the Georgia music scene that spawned R.E.M. and the B-52's, among the other bands that broke into the mainstream during that decade. As a result, the Indigo Girls' first major-label album, released in 1989, was a commercial and critical success based on the strength of what would eventually become the duo's signature song, "Closer to Fine."

While other female singer/songwriters have largely faded from the public consciousness, the Indigo Girls remain relevant as they approach their 25th anniversary playing together Indigo Girls. That can be attributed to both the power of their rock-influenced folk and their activism; Saliers and Ray have become outspoken in support of lesbian, gay, bisexual and transgender issues, as well as environmentalism.

They also promote community radio stations. Ray's 2008 song "SLC Radio" supported KRCL with lyrics such as:

"I'm pulling into the LDS nation looking for a community station

'Cause I've heard about the kids in Salt Lake City

And how they fight to be set free and how they fight

For you and me radio, radio community."

While Ray has released four solo albums, Saliers hasn't recorded any solo projects. "I don't feel a burning desire," Saliers said. As in other creative partnership, the Indigo Girls admit to experience some conflicts when they come together to record an album, but such issues are resolved under the rule of "whoever writes the song has the veto power."

Their partnership is exemplified in the pair's latest album, March's "Poseidon and The Bitter Bug." Each woman wrote five songs on the album, the first album released by the duo's imprint, IG Recordings. "We ran our course with record labels," Saliers said. "I don't think they could sell more albums than we could [alone]."

And so far, that business tactic appears to have paid off, as the new CD is the highest-charting album for the Indigo Girls in 12 years.

The duo invited Colorado-based folk singer-songwriter Gregory Alan Isakov to open for them on this tour. Ray became a fan of Isakov when he opened up up-and-coming folk juggernaut Brandi Carlile's tour earlier this year.

Isakov, whose great grandfather emigrated from Lithuania to South Africa, lived in the latter country until his parents, disgusted by Apartheid policies, moved to the United States in 1986. He grew up in Philadelphia, but settled in Colorado after he went to horticulture school at Boulder's Naropa University. After graduating, he worked on a farm for seven years. "I had a lot of time to think," he said of the experience.

As a farmer, Isakov often worked for days without seeing another person, and songs about landscapes percolated in his mind. Since he began his full-time musical career, he has become a Colorado favorite, and is excited about his first performance in Utah, a state he has only driven though.