Guitarists Kaukonen and Bromberg at the Keswick

America may be a musical melting pot, but most of its musical genres— say, bluegrass, the blues, jazz and country— share something in common: the guitar, a remarkably versatile instrument.

The guitar's six (or 12) strings make it suitable for playing both melodies and chords; unlike wind instruments, the guitar lends itself to an accompaniment for one's singing; and unlike the piano, it's eminently portable. For all of these reasons, the guitar was embraced by musicians of all races and ethnicities and put to work in a variety of styles.

Of course, the guitar derives its significance from the men and women who play it. By listening to each other and playing together, these guitarists function as the bumblebees that cross-pollinate musical genres.

Branches of the blues

"An Evening of Duets" at the Keswick Theater evoked some of this history. The concert brought together two finger-picking greats, Jorma Kaukonen and David Bromberg. Both men started playing guitar more than 50 years ago, and both began by playing the blues. Over the ensuing half-century, each has explored numerous branches of the tree rooted in the blues.

Kaukonen first gained fame as a member of the psychedelic rock band Jefferson Airplane, but he's probably best known for that group's acoustic spin-off, Hot Tuna. Bromberg, a native Philadelphian, started as a sideman (to Janis Joplin, Bob Dylan, Willie Nelson and many others) before embarking on a career as solo performer, front man and impresario.

Kaukonen opened the concert with Barry Mitterhoff, a mandolinist of astonishing skill. The pair played bluegrass-tinged duets, both men swapping off complex melodic lines and Kaukonen doing the singing.

Mandolin in the mix

Bromberg then joined them onstage, and blues replaced bluegrass as they performed classic songs like "The Things That I Used to Do." Although Mitterhoff's mandolin isn't a common instrument in blues, it worked well with the two guitars. Bromberg and Kaukonen took turns on vocals, with Mitterhoff adding some harmony from time to time.

After the intermission, Bromberg did a solo set that included a tribute to his mentor, the Reverend Gary Davis ("Maple Leaf Rag," "Just a Closer Walk with Thee"); a virtuoso version of Ray Charles's "Fool for You"; and a couple of tunes on Dobro guitar. Kaukonen and Mitterhoff subsequently returned for more blues, including another Reverend Gary Davis tune ("Death Don't Have No Mercy") and "Statesboro Blues."

What to play next?

The mood throughout was relaxed and casual. The men hadn't finalized a set list in advance, and they paused frequently between songs to decide what to play next. ("Thank god we're not over-rehearsed," Kaukonen joked.) The spontaneity served the music well; after all, both blues and bluegrass are genres of improvisation.

By including the mandolinist, the two guitarists kept their program from getting too predictable. They broke no new ground, but it was a treat to hear classic blues played so well by three men who clearly love the music at the root of their musical lives.