Sometimes, the best bands happen by accident. They begin, perhaps, as a side project or impromptu jam session between friends or strangers, where everything suddenly, magically, goes right. And then never stops going right.


Singer-guitarist Ry Cavanaugh pegs not one but two moments when his Cambridge-based outfit, Session Americana, more or less began: The first was a boisterous Camp Street Studio party where ex-Treat Her Right harmonica master Jimmy Fitting sat in with the freewheeling roots outfit the Benders. The second happened soon   after. Cavanaugh and singer-songwriter Jabe Beyer were performing at Toad together and had exhausted their set list by midnight. So Cavanaugh placed three microphones on a nearby table, threw caution to the wind, and started brainstorming tunes with Beyer and the band. The idea for Session Americana, as Cavanaugh put it at the time, was to capture "the vibe of an Irish session but have it be American music, just guys sitting around playing."


That was three years ago, but it might as well have been a lifetime. A musical collective formed around the nucleus of Cavanaugh, Fitting, and four other of the city's busiest, most experienced musicians -- Billy Beard, Dinty Child, Kimon Kirk, and Sean Staples -- swapping musical tunes and tales around a tabletop, Session Americana has become one of the region's most popular guaranteed good times.


What started as a modest Sunday-night residency at Toad, a tiny nightspot in Cambridge's Porter Square, has exploded into a ritual and expanded to dates performing at festivals and showcase gigs throughout New England and New York City. Two years ago, the group grabbed a Boston Music Award as "Best Folk Act" -- although anyone who's seen or heard them knows they're also so much more . ("At the end of the day," says Cavanaugh proudly, "we're a rock band.")


"At the end of that first two-year run playing every Sunday night at Toad, we knew something was going on," says Beard of the quickly multiplying crowds. "It was crazy -- you couldn't get in the place. And it's been like that ever since."


The band has just released its second CD, "Beer Town: The Tabletop Collective, Volume 3," on Cambridge's Hi-N-Dry label, and is at the Lizard Lounge every Tuesday through May. "It's so much fun and so simple and effective," says Cavanaugh of the band's open-ended approach onstage, where anything goes musically. "Sometimes, when you're in a band, you walk out onstage and you strum a chord, and you're already in a [genre] category for people. With us, there's no ego facing outward because we're all facing each other. And we really like our audience -- they have a good time and are all ages, and there are a lot of girls. What more can you ask for?"


Although its eight tracks clock in at just over 30 minutes, "Beer Town" is an eclectic, swinging tour de force that leaves the listener with the same question: What more could one ask for -- except, perhaps, for more terrific tracks such as "A Month of Sundays," a Jayhawks-esque country rock number written by Kirk and dappled with harmonica, mandocello, and field organ. Or their cover of Jimmy Ryan's showpiece ballad, "John Brown," which closes every Session show. Or Child's facetious tribute to the gods and goddesses who bring us the precious amber elixir that inspires the title track.


"We thought that would be a good topic," says Cavanaugh with a laugh. "We kind of wanted to carve this record out as not a follow-up to the family-oriented record [the double-disc 'Tabletop People Vol. 1 & Vol. 2'] that we made before. I figured that with 'Beer Town,' you probably wouldn't have too many parents of young children picking that up to put in the car. Although my 3-year-old son absolutely loves the song 'Beer Town' and sings it perpetually."


During the sessions for "Beer Town," the band had also been recording with singer-songwriter Rose Polenzani (who also appears as a guest on the new album) at Hi-N-Dry, the label's loft space-cum-recording studio that once belonged to late Morphine leader Mark Sandman. Electric guitars happened to be the order of the day, so instead of reconfiguring the room, Session Americana went with what was there. That embrace of the unknown and free-spirited sense of adventure is, Cavanaugh believes, Session Americana's greatest asset.


"I think that's the great strength of the band -- that you can do whatever you want with it," he says. Cavanaugh offers one caveat, however. "But you gotta keep the table. You're stuck with that."