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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.
As the instrumental proficiency of pop and rock musicians improved in the 1960s and early 1970s, onstage jamming--free improvisation-- also became a regular feature of rock music; bands such as Pink Floyd, Cream, the Jimi Hendrix Experience, Deep Purple, The Who, the Grateful Dead, AC/DC, Led Zeppelin, Santana, King Crimson and the Allman Brothers Band would feature live improvised performances that could last anywhere between 10 and 20 minutes. However, they can be shorter on the recorded version.
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.
- The Soundtrack for Tonite Let's All Make Love in London features 2 improvised jams titled "Interstellar Overdrive" and "Nick's Boogie" by early Pink Floyd. The latter is complete improvisation around the beat of the drums provided by drummer Nick Mason.
- The farewell Last Waltz concerts led by The Band in 1976 included two late-night jam sessions featuring Neil Young, Ronnie Wood, Eric Clapton, Ringo Starr and others along with members of The Band. These performances were not included in the film or original recordings of the concert officially released for the first time as part of a 2002 box set.
- The third disc of George Harrison's 1970 solo album All Things Must Pass, titled Apple Jam, features a variety of songs by session musicians who contributed to the LP.
- The Velvet Underground's "Sister Ray", "Some Kinda Love" and "Foggy Notion", had extensive live versions.
- Nirvana's improvised hidden track "Gallons of Rubbing Alcohol Flow Through the Strip" from the album In Utero (1993), and 'semi-improvised' hidden track from Nevermind (1991).
- Liquid Tension Experiment's "Three Minute Warning" from the album Liquid Tension Experiment was a 28 minute long improvised jam, the result was so unexpectedly fluent that the members decided to include it on the release.
- "Bluesbreaker" by Brian May and Friends recorded during the Star Fleet Project and featuring Eddie Van Halen, Phil Chen, Alan Gratzer and Fred Mandel.
- "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.
- Ex-Red Hot Chili Peppers guitarist John Frusciante, Fugazi bassist Joe Lally and Red Hot Chili Peppers guitarist Josh Klinghoffer formed a brief project named Ataxia, releasing two albums; Automatic Writing and AW II; with lengthy tracks based largely around improvisational guitar set against a steady, unchanging bassline and drum beat.
- 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 guitarist Angus Young. Songs often jammed to are "Let There Be Rock", "Bad Boy Boogie", and "The Jack."
More recently, the jam band has become a genre unto itself; following in the footsteps of jam band originators the Grateful Dead, performances by groups including Phish, moe., Umphreys Mcgee, and Widespread Panic feature extended improvisational sessions. Other bands, such as the Red Hot Chili Peppers also regularly perform live jam sessions. Progressive rock band Coheed and Cambria often end shows with a jam session to their song "The Final Cut" with different instruments.
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.
- "Really The Blues" by Mezz Mezzrow
- Jam band
- Jamming (dance)
- Free improvisation
- Free jazz
- Freestyle rap
- Bluegrass music
- Blues scale
- Scat singing
- Session musician and Irish traditional music session
- Underground music
- Collaborative website for jam session
- Finding Bluegrass (and acoustic folk) jam sessions
- Jazz jam sessions in Tokyo, Japan