Back before Johnny Rotten and the Sex Pistols, before Pat Boone, before "By the Light of the Silvery Moon," popular music concerts were always jam sessions.

Tunes were introduced with "Let's try . . ." or "Do we know . . . " or "How about . . . " Anyone with a fiddle, banjo or jew's harp was welcome in the band.And this summer, dozens of Salt Lakers are bringing back the spirit of those old-timey, good-timey sessions. Once a week bluegrass buffs jam at Sugarhouse Park. Other groups meet regularly at the Silverado in Carriage Square for bluegrass; some meet at private homes and in music stores. In the spring there was Irish music at Murphy's. On June 22 there'll be a big jam session at the Great Salt Lake Guitar Co. in Provo.

"People go to jam sessions because it's an opportunity to exchange some musical ideas and relax in a nice atmosphere," says Jay Toups of Acoustic Music. "Each jam session has its own focus. At bluegrass sessions, for instance, the players who show up usually know the repertoire and can play the tunes."

Last week an "old time acoustic music" session was held at the home of Trish Barrett of Salt Lake City. "Old time" music predates bluegrass and is not as hard-driving. There's a gentle, tumbling feel to most of the songs, a light, tripping sound that smacks of British folk music. It's also less demanding than other styles. Any person with a flat-pick strum and at least three chords in their arsenal can usually do just fine.

At the old-time sessions, players usually sit in a circle and go around the ring a person at a time, each one suggesting a song. If the number's new, it takes about 20 seconds to pick up the chord progression and join in. Some of the songs run for eight or 10 minutes as the group changes tempos, adds a few variations or changes keys. Some of the "singers" - as the vocal songs are called - might be just a couple of verses and done.

Some weeks a half-dozen people will show up, and three of them will be fiddlers. The next week it will look like a back-porch orchestra with string bass, triangles, spoons, concertinas, banjos, guitars, mandolins and autoharps.

"Jamming has been going on for as long as I've been involved in the music scene here, and that's about 20 years," says Hardin Davis, a local musician. "Lots of local bands have come out of jam sessions. That's how most musicians around here have met each other. When people start to form bands they get together and not jam but `practice.' The official word comes into it.

"As for the local, traditional music scene, I see several directions. We're getting a lot of interest in jam sessions right now, but there seem to be fewer places for people to perform. Perhaps one is the function of the other."

If you're interested in jamming, or just want to show up and listen for a while, you might call Acoustic Music at 531-7066. The little store - just off Ninth East - has pretty much become the nerve center for local jammers.