WHEN FORMER Toad the Wet Sprocket frontman Glen Phillips wanted to work up some new tunes, he thought he'd call some friends over to his California garage and lay down some tracks on his home studio gear.

The fact that his called-upon pals happened to be the masterful acoustic band Nickel Creek led to much interest from record labels and to those garage sessions' finally being released a few weeks ago by Sugar Hill Records (Nickel Creek's label). But why did four years have to pass between recording and releasing the music? "That delay was a combination of legal stuff and timing issues," Phillips explains. Both he and Nickel Creek were releasing their debut CDs in 2000 and didn't want to take attention away from those recordings. Then there was some legal limbo that no one wants to talk about. And some question about what to call the collaboration. They decided to call it what it was: a Mutual Admiration Society.

"We'd been longtime Toad fans," says Nickel Creek fiddler and singer Sara Watkins. "They were one of the first rock bands we all really got into, to the extent of being able to play the entire 'Coil' record straight through." She laughs. "We were kind of dorks about it."

A friend of the band heard them strumming through "Coil" backstage at a music festival several years ago, and passed that fact along to another friend, who happened to be Phillips. The two artists made contact, and it led to Phillips singing a tune on Sean Watkins's first solo recording (Sean is Sara's brother and Nickel Creek's guitarist).

"Then we were all at [L.A. club] Largo in early 2000, I guess, and played with Glen there," Sara says. "We just had a really great connection, and it just kept going from there." That year, they played together a few more times and decided to document the growing mutual affinity.

"They had a few days break on a tour, so I asked them to come on over," Phillips says. "We had one day to choose the songs, then three days to record, pretty much learning the songs as we recorded them." The results are what you'd expect: lovely, melodic tunes played with sheer instrumental brilliance. It's mostly a quiet affair, which will surprise anyone who first hears these songs played at the Birchmere on Wednesday and Thursday, when the Mutual Admiration Society comes to town. That's because the quartet has been augmented with a couple of guys who will surely beef up the sound: drummer Pete Thomas -- formerly of Elvis Costello's band the Attractions -- and bassist-keyboardist-mandolinist John Paul Jones, the former Led Zeppelin bassist.

So let's recap: Led Zeppelin's bassist, Elvis Costello's drummer, Toad the Wet Sprocket's frontman and the hottest bluegrass act on the planet. Can you come up with another band with quite that varied a pedigree? Didn't think so. So how did this happen? "I used to play mandolin quite a bit with Zeppelin, you remember," Jones says. "I left it for quite a while, but then heard Alison Krauss on the radio here in England, and I was enchanted with the playing of her mandolinist, Adam Steffey. A friend told me that if I liked Adam, then I should check out Chris Thile." Thile is the third member of Nickel Creek and is widely considered one of the best mandolin players in history. Jones quickly became a fan. He caught the group live a few times in the United Kingdom "and got to chatting with them."

His interest piqued, Jones got online and booked himself a cabin in North Carolina in March this year and bought himself a ticket to Merlefest, perhaps the premier festival for bluegrass, old-time country, Americana and new acoustic music. "I really liked the look of the lineup and decided to bring my mandolin," Jones says. "It was incredible. The people were so nice, there were so many amazing musicians, like Gillian Welch and David Rawlings, like Tim O'Brien, Nickel Creek, and I got to play with a few of the acts. I just hung out a bit and tried to soak in as much of the music as I could."

When he got back to England, Jones got a call in his office. Would he like to play mandolin and bass on a short tour with the Mutual Admiration Society? "Well, that sounded like a hell of a lot of fun!" he says.

But wouldn't it be a weird step for a man who anchored one of the greatest rock bands in history? "I've always been like that. I have a very varied music background," Jones says. "My parents were in variety, which I guess would be the equivalent of vaudeville in the States. I used to tour around with them, so I'd see Chinese acrobat musicians, Hungarian bands, different things every night. Then I discovered rock 'n' roll, then the Everly Brothers, and I would work out the [absent] third harmony on all their records." Jones joined a rock band and toured all over England, but his multiple talents led him to becoming a bass player and arranger on recording sessions. "I was one of the younger guys who knew about rock 'n' roll, knew about soul music. I could hear all the parts, so I became an arranger and got a lot of work quite quickly. Horn charts, string arrangements, vocal harmonies, that sort of thing.

In fact, Jones got too busy. "I was going mad doing too much work, and it was my wife who read in a magazine that Jimmy [Page] -- who'd left session work to join the Yardbirds -- was forming a band. 'Give him a call,' she said. 'You're going crazy in the studio.' So I did, thinking I would do that a couple of years, then settle down and write film scores." He laughs at the naivete of his younger self. As Led Zeppelin, Jones and Page, along with singer Robert Plant and drummer John Bonham, became one of the most important bands of all time. Since Zeppelin broke up in the early '80s, Jones has been a producer, has released two solo records and has toured with singer Diamanda Galas. "She got me at it," Jones says. "She reminded me how fun it is to play in front of people."

Another of the musical pleasures Jones has rediscovered is jamming. "Especially at Merlefest, in the bluegrass world, everyone just pulls out an instrument and says, 'Who's got a song?' That doesn't really happen in rock 'n' roll. And remember, I'm a bassist and organist, and hauling gear for those things requires trucks. With a mandolin, you become a hell of a lot more portable."

So Jones is carrying his mandolin onto the Mutual Admiration Society tour bus, a fact that has Sara Watkins giddy. "Imagine the stories, for crying out loud!" she yells. "The stories! The stories! I'm going to get a Minidisc player and hide it somewhere on the bus and not tell anyone, and record everything these guys say. My video camera isn't working too well. I think I need to buy another one before we go out."

Jones's presence in the band also has Phillips a little scared. "I'm terribly nervous, frankly," he says. "I'm not usually, but they're all such great players, with such strong musical pasts. There are certain things I'm good at, and certain things I'm less good at, so I'm really trying to rise to the occasion. I'm stepping up to the plate as a musician more than I have in years and years and years. And I have to say, it's been wonderful to feel so challenged. I'm really working at being at my best."

Though it was Nickel Creek that recruited Jones, it was Phillips who got Thomas to sign on as drummer. "My producer had gotten him to play on my last album, and we got along really well. The funny thing is, I was hesitating to ask, because you figure Pete is going to be busy, you figure John Paul Jones has gotta be busy! But you just have to ask. They might say yes. We're lucky they both did!"

Phillips admits that the paycheck might be smaller than Jones and Thomas might be used to, but "this is not something we're doing for cash, but for the love of it. Everyone seems really happy to be doing it. Being in successful bands can get people very jaded, to feeling very entitled. This is a good chance to remember what an incredible gift it is to be able to play music for people. And in this case especially, to play night after night with such incredible musicians."

For people looking for a second volume from the Mutual Admiration Society, Phillips says that the group will be recording every show on this tour and that a live CD is the likely outcome. "We're playing Zeppelin's 'Gallows Pole' on the tour, and also 'Going to California,' (that one as an instrumental), and we're doing some Toad the Wet Sprocket tunes, some Nickel Creek tunes, so it's not just songs off the M.A.S. record. There will be lots of musical surprises."

MUTUAL ADMIRATION SOCIETY -- Appearing Wednesday and Thursday at the Birchmere. * To hear a free Sound Bite from Mutual Admiration Society, call Post-Haste at 301-313-2200 and press 8121. (Prince William residents, call 703-690-4110.)

I love you, you love me; we're the Mutual Admiration Society: Glen Phillips, from left, of Toad the Wet Sprocket joins creative forces with Nickel Creek's Sara Watkins, Sean Watkins and Chris Thile.