When is the sum of the parts of a musical quantity far greater than the whole? When Kelly Willis and Bruce Robison unite as an ongoing musical act.

After all, their own individual artistic achievements already sum up to impressive totals. Willis has reigned as a critically acclaimed roots music singer since her recorded debut some two decades ago, and is recognized as a new country and Americana pioneer. Her ever-broadening artistic range and gifts as both a canny writer and interpreter of songs have consistently continued to wow her many fans and music critics alike thanks to her finesse, smarts and irresistible charm. Robison has made his mark as both a writer of indelible smash #1 hits like “Wrapped” (George Strait), “Angry All The Time” (Tim McGraw & Faith Hill) and “Travelin’ Soldier” (Dixie Chicks) and an artist in his own right who boasts a series of eight albums rich with his laconic yet potent grace and emotional dead aim as a songwriter and singer.

But as this couple in life, love and parenthood now unite as a band all its own, it’s more about the promise and thrills of the new than their considerable solo legacies and the laurels they’ve already earned. “We’ve been playing together occasionally over the years,” explains Willis. “And it’s always been really special. So we just thought we’d like to try a real collaboration where we pull from each other’s strengths and create a new entity apart from our separate ones.”

“It’s been something that I’ve wanted to try for a couple of years,” adds Robison. “At some point you decide: let’s see what this is like if we don’t keep it at arm’s length.”

What the collaboration will become is an open proposition rife with exciting possibilities and potential, as anyone who knows the music they’ve both created already is sure to agree. As Willis and Robison now embark on a new act together as their primary creative endeavor they will continue to make the most of their own separate musical identities. And the one double-down sure thing is that they will follow the music and their now united muses to delightful and engaging places.

Married for some 15 years and the parents of four children, “We didn’t want to be considered a duo before we both made our own names and paths in our separate careers,” says Willis. After releasing a holiday EP together in 2003 that they expanded into an album in 2006 — described by All Music Guide as “elegant and concise… with plenty of heart and soul… it delivers equal portions of taste, passion and great music” — their annual Christmas season concerts have become a Texas Yuletide tradition. Willis has recorded a number of songs by Robison. And as he notes, “Every song of mine that did well for me had Kelly singing on it.” Now their mutual admiration society becomes a full artistic partnership.

Willis, who was born in Oklahoma, has been singing professionally since her teens in the Washington, DC area. Her group Kelly & The Fireballs relocated to Austin in the late 1980s after she finished high school. Willis was signed soon after to a recording contract with MCA Records after being touted to the label by Nanci Griffith, and released three highly praised major label albums: Well Traveled Love (1990), Bang Bang (1991) and Kelly Willis (1993). After a 1996 EP on A&M Records, Fading Fast, she chose to pursue her career as an independent artist and recorded her fourth album, What I Deserve, on her own. It landed Willis a deal with Rykodisc and was hailed by Time as “the smartest, most consistently worthwhile country CD” issued yet that year (1999). She followed it with Easy in 2002 and Translated From Love in 2007. As All Music Guide notes, “Willis is the darling of alt country fans and NPR listeners, and each recording has received more platitudes than the one before.”

She also appeared in the Tim Robbins film Bob Roberts and was heard singing the Dave Alvin/John Doe song “Little Honey” in the hit movie Thelma and Louise. In addition to songs she has written on her own, Willis has collaborated with such other writers as Gary Louris (Jayhawks), Chuck Prophet, Jules Shear, John Leventhal, and hit country songwriters Kostas and Paul Kennerley. She has also put her own distinctive interpretive stamp on material by John Hiatt, David Bowie, Nick Drake, Marshall Crenshaw, Joe Ely, Kirsty MacColl, Paul Westerberg, Paul Kelly and others. PopMatters rates her as “one of Austin’s finest singer-songwriters… who may possess the most heartfelt voice in country music.” For as Amazon.com notes, “Willis has everything: a flawless, classic country voice; a dreamy face; [and] a shrewd eye for catchy but complex material.

Bruce Robison hails from Bandera, TX, known as “The Cowboy Capital of the World,” and started playing music as a teen in garage bands. He began his career in the early 1990s when his brother Charlie — also a noted singer, songwriter and recording artist — persuaded Bruce to drop out of college and move with him to Austin to play music. After a stint in the band Chaparral and a short-lived collaboration with brother Charlie, The Weepers, Robison stepped out on his own as a solo singer and songwriter with his self-titled 1995 debut album on a small local indie label that featured two of the later #1 hits he wrote. He issued Wrapped on his own in 1997, which was then picked up for wider release the following year by Sony Nashville when he signed with its Lucky Dog imprint. Two other major label records followed: 1999’s Long Way Home From Anywhere and a concert recording with his brother and labelmate Jack Ingram, Unleashed Live.

Robison began making records again as an independent with 2001’s Country Sunshine. He hit a prolific streak of albums with Eleven Stories (2006), It Came From San Antonio (2007), The New World (2008), 2009’s The Greatest, a rerecorded collection of his top hits and favorite compositions, and the Rodney Crowell-produced From the Top in 2010. Though touted as “the likely future dean of Texas songwriters” by No Depression, he has also cut songs by such diverse artists Webb Pierce, Cat Stevens and The Grateful Dead as well as some of his talented Austin friends. In addition to his #1 songs, he also penned a Top 10 hit for Strait, “Desperately,” with his pal Monte Warden, and has enjoyed covers of his songs by Lee Ann Womack, Garth Brooks, Aaron Watson, Gary Allen, Allison Moorer, Charlie Robison and other artists. All Music Guide hails his “achingly beautiful lyrics,” and as No Depression notes, “the quality of his craft coupled with his affable charm make him impossible to dislike.”

“There’s so much good work for both of us to pull from,” notes Robison. “Every show can be different. There’s a lot of our material that can be worked into it.”

But the thrust of their new creative partnership as their core artistic venture is to develop a musical quantity that is entirely new and utterly its own. It’s all about “finding the right songs and the right vibe — anything that serves that moment of playing the music and having it feel right,” says Bruce. They are also gathering a crew of musicians to meld into a genuine band and make this venture distinctive and different from all they’ve done before well as rich with collaborative spirit.

The excitement that Willis and Robison both exude as they embark on the next chapter of their musical artistry feels palpable. As Kelly enthuses, “It’s rare to find someone who you really click with musically. We don’t want to squander that gift. It just feels right for us to make the most of it now.”

“Yeah, it does feel like the time is right for us to do this together,” agrees Robison. “Kelly is such an amazing singer, and she makes a song sound like it ought to sound. With the songs that she writes and her instincts about the big stuff, it’s going to be a lot of fun.”

And not just for Willis and Robison, but their fans as well. After all they’ve both accomplished on their own, the union augurs even greater and more wonderful sounds to come. And one thing is for sure: listeners will enjoy eternally resonant modern yet rooted country music to be savored and treasured like a bond of true and abiding love.