A rhythm section includes the musicians in a jazz or popular music band or ensemble who establish the rhythmic pulse of a song or musical piece and lay down the chordal structure. The term "rhythm section" may also refer to the instruments in this group.

http://en.wikipedia.org/wiki/Rhythm_section


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Note added at 15 mins (2008-02-17 22:11:40 GMT)
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Ovaj pojam ne treba mešati sa pojmom drum and bass, ovo potonje je vrsta muzike.
Drum and bass (commonly abbreviated to d&b, DnB, dnb, d'n'b, drum n bass and drum & bass) is a type of electronic dance music also known as jungle which emerged in the early 1990s.
http://en.wikipedia.org/wiki/Drum_and_bass

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Note added at 25 mins (2008-02-17 22:21:24 GMT)
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The core elements of the rhythm section are the drums and a bass instrument (either double bass for Swing or Bebop jazz or a bass guitar for pop, fusion, R&B and funk). Both instruments lay down the pulse for the music, and the bass lays down the harmonic foundation with a bassline. Most groups also use chordal instruments in their rhythm sections, such as rhythm guitar, piano or other keyboard instruments, such as the electric piano, Hammond organ, or vibraphone.

Swing big bands, Afro-Latin bands, and jazz fusion bands often use auxiliary percussion in the rhythm section. New Orleans or Dixieland Swing bands may use a Banjo in the rhythm section.

http://en.wikipedia.org/wiki/Rhythm_section