Nearly a quarter-century has passed since the Indigo Girls became a Grammy-winning household name synonymous with beautiful harmonies, thoughtful lyrics and an activist folk-rock attitude.

The duo's passion for meaningful music hasn't faded in the years since. Every album is an evolution of a collaboration that began as teens for Amy Ray and Emily Saliers.

"We started when we were super-young," Ray says. "We were doing cover songs and kind of learning how to sing together and having a good time."

The two taught each other as they went along, developing the harmonic style that defines the Indigo Girls.

"For me, I knew I was going to be a musician, I knew it would be a career, but I wasn't sure how I was going to accomplish it," Ray says. "Although we've grown and hopefully our songwriting has evolved, there's still the essence of what we had when we started, which was just this willingness to discover and to not get stagnant."

Stagnation doesn't seem to be a serious threat for the pair, who have produced 14 studio albums as well as a handful of live and greatest hits albums. The latest is "Beauty Queen Sister," and Yorkers will get the chance to hear songs from the album live alongside their old favorites when the Indigo Girls play at the Strand-Capitol Performing Arts Center on Thursday.

"It was pointless to do it if it was just going to be the same thing over and over again," Ray says of the importance the duo places on stretching themselves with every piece of music they produce. "That core belief carried through and informed everything that we do. ... I think it's what's kept us together."

Solving the puzzle: She treats the craft of songwriting like a crossword puzzle, she says, shaping the story of a song until each word joins in a compact whole bursting with meaning. The melody, she confesses, is still the hardest part of the process for her.

"That's the thing that I'm working on more nowadays. I don't necessarily have the magic touch with it," she says. "I'll change the chorus or the bridge or move things around" and record the piece with several variations.

"I like to exhaust the different possibilities," she says. "It's like when you get the eyes checked, over and over again until there's the one when it's in focus."

The fans: The longevity of fans' devotion is one way Ray and Saliers know they're seeing clearly.

"We have a really great fanbase, small but mighty," Ray says. "Life gets so busy, and we really appreciate the people that maintain that relationship" with the group and the music.

"It inspires you to create and keep trying to write good songs," she adds.

For their tour stop at the Strand, the Indigo Girls will be bringing along new friends in the form of an all-male trio that provided backup vocals on "Beauty Queen Sister."

"The Shadowboxers are a great new band, so we're excited about that," Ray says. "They play a set, and then we get up and they back us up and we all play together."

See the show

The Indigo Girls will give a concert at 7:30 p.m. Thursday, June 27, at the Strand-Capitol Performing Arts Center, 50 N. George St., York.

The Shadowboxers will open the show.

Tickets are $42-$50.

For more information, call (717) 846-1111 or visit www.strandcapitol.org.

- Mel Barber at mbarber@yorkdispatch.com.