A jam session is a musical event, process, or activity where musicians play (i.e. "jam") by improvising without extensive preparation or predefined arrangements. Jam sessions are often used by musicians to develop new material (music), find suitable arrangements, or simply as a social gathering and communal practice session. Jam sessions may be based upon existing songs or forms, may be loosely based on an agreed chord progression or chart suggested by one participant, or may be wholly improvisational. Jam sessions can range from very loose gatherings of amateurs to evenings where a jam session coordinator acts as a "gatekeeper" to ensure that only appropriate-level performers take the stage, to sophisticated improvised recording sessions by professionals which are intended to be edited and released to the public.
The origin of the word "Jam Session", came about when white and black musician would congregate together, in the 20's, after their regular paying gigs, to play the jazz they couldn't in the "Paul Whiteman" style bands. Bing Crosby would attend these sessions, and the jazz cats always said he was "jammin' the beat", since he would clap on the one and the three. So these sessions became known as "Jam Sessions".
The New York scene during World War II was famous for its after-hours jam sessions. One of the most famous was the regular after-hours jam at Minton's Playhouse in New York City that ran in the 1940s and early 1950s. The jam sessions at Minton's were a fertile meeting place and proving ground for both established soloists like Ben Webster and Lester Young, and the younger jazz musicians who would soon become leading exponents of the bebop movement, including Thelonious Monk (Minton's house pianist), Charlie Parker, and Dizzy Gillespie. The Minton's jams were legendary for their highly competitive "cutting contests", in which soloists would try to keep up with the house band and outdo each other in improvisation skill.
Some notable recorded jams and jam-inspired performances in the rock idiom:
The bonus CD of the 25th anniversary of the album Layla and Other Assorted Love Songs by Eric Clapton's early 70s band, Derek & The Dominos includes a number of long improvised jams between members of the group and other musicians, such as The Allman Brothers Band following the historic first meeting between the two groups earlier that day. As a result of this jam, guitarist Duane Allman was invited to join the Dominos after having recorded only three songs, and he made a major contribution to the resulting LP.
"Voodoo Chile", a track appearing on Jimi Hendrix's album Electric Ladyland, which would prove the basis for one of Hendrix's best known songs - Voodoo Child (Slight Return) - is a 15-minutes-long blues-rock piece with various improvised sections.
The Who's "How Can You Do It Alone" from Face Dances began as an onstage jam during performances on their December, 1979, tour in the United States. In these performances, Pete Townshend, doing the lead vocal, would improvise the lyric each time out. Furthermore, Townshend's solo song "Dance It Away" was borne out of shorter jams on this same tour.
During AC/DC's live act, several songs are extended into 10-25 minute jams, sometimes including a strip tease by lead guitaristAngus Young. Songs often jammed to are "Let There Be Rock", "Bad Boy Boogie", and "The Jack."
During Eagles' live concert's for their Hotel California Tour in 1977, they would jam for the intro of Witchy Woman, causing it to exceed its original length to almost 10 minutes.
Bluegrass music also features a tradition of jamming. Bluegrass jams happen in the parking lots and campgrounds of bluegrass festivals, in music stores, bars and restaurants and on stages. Bluegrass jams tend to be segregated by the skill level of the players. Slow jams for beginners provide an entry point. Open bluegrass jams are open to all comers, however, the players in an open jam will expect a certain level of proficiency. The abilities to hear chord progressions and keep time are prerequisite; the ability to play improvised leads that contain at least a suggestion of the melody is desired. Jams that require advanced musical proficiency are generally private, by-invitation events.