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A rhythm section is a group of musicians within an ensemble who provide the underlying rhythm and pulse of the accompaniment, providing a rhythmic reference for the rest of the band. Many of the rhythm section instruments, such as keyboards and guitars, play the chord progression upon which the song is based.
The term is common in modern small musical ensembles, such as bands that play jazz, country, blues, and rock. In modern rock music, a rhythm guitarist specializes in rhythmic and chordal playing (as opposed to melodic and leading), sometimes simply repeating quaver (eighth-note) power chords, or strumming open chords.
A typical rhythm section comprises a keyboard instrument and/or one or more guitars, a double bass or electric bass (depending on the style of music), and drums (usually acoustic, but in some post-1980s styles, the drums may be electronic). The guitars may be acoustic or electric, depending on the style of music.
The instrumentalists used in a rhythm section vary according to the style of music and era. Modern pop, rock and jazz bands rhythm sections typically consists of a drummer, a bass player, and one or more players of chordal instruments (e.g., a pianist, guitarist, etc.). The term rhythm section may also refer to the instruments in this group (named collectively the "rhythm section instruments". In music industry parlance, the amplifiers and some of the instruments are nicknamed the "backline." The backline typically includes large and heavy items that are hard to transport, including large bass amplifiers and guitar amplifiers, the drum kit (possibly minus the cymbals), a Hammond organ, stage piano, and a keyboard amplifier. Even when a venue or festival provides a "backline", musicians must still supply some instruments themselves, such as guitars, an electric bass, and, in some cases, the cymbals and/or the snare drum. The venue informs musicians about which instruments are supplied at in the backline for a specific concert.
Although rhythm sections spend much of the time providing backing parts for songs, in some cases, they provide other musical roles. In some songs or styles of music, instruments from the rhythm section may play soloistic roles on occasion (e.g., improvised solos or solo breaks) or play a melodic role (e.g., a guitarist may play a lyrical countermelody behind a singer). Since rhythm sections are generally providing the background music for lead instruments and solo singers, rhythm sections are typically not as prominent as a singer or soloist. However, since rhythm sections provide the underpinning for a good performance by the lead instruments and vocalists, good rhythm sections are valued in the music industry.
In the case of swing bands, the classic rhythm section comprises a quartet of electric guitar, piano, double bass, and drums; a noted example is that of the Count Basie Orchestra with Freddie Green, the Count, Walter Page, and Jo Jones. Earlier jazz bands had used banjo in place of guitar, and other bass instruments such as the tuba for recording purposes prior to the advent of microphone technology in studios.
As bebop evolved, smaller jazz groups dropped the guitar entirely, and many free jazz ensembles dropped the piano as well. Auxiliary percussion such as claves, bongos or maracas can also be used, especially in music influenced by strains from Latin America such as salsa and samba. In theory any instrument or instruments can provide a steady rhythm: for example, in the trio led by Jimmy Giuffre the late 1950s, the clarinet, valve trombone and guitar all switch between lead and supporting roles.
In the 1970s, with the advent of jazz rock and jazz fusion, there was a major change in the composition of the jazz rhythm section. The upright bass was replaced with the electric bass, which was much easier to amplify to stadium-filling volumes using large bass speaker cabinets and amplifiers.
As well, the main chordal rhythm instruments became electric instruments such as the Rhodes electric piano or clavinet, often run through effects units such as fuzz, phasers, or wah-wah pedals. The jazz fusion rhythm section followed the lead of the rock rhythm sections of the era, and used huge banks of speakers and powerful amplifiers to create a massive sound. In the later 1980s and subsequent decades, jazz fusion bands such as the Chick Corea Elektric Band used synthesizers in the rhythm section, both for chordal accompaniment and for synth bass parts.
R&B, rock and pop 
R&B and Rock and Roll groups in the 1950s emphasized rhythm, their bands generally consisting only of the standard swing band rhythm section of guitar, piano, bass, drums supporting a vocalist, and in some cases abandoning the keyboards altogether. The bass guitar took over from the double bass in the 1960s, and as the music progressed into the 1960s, the term "rhythm section" as used in a pop music context often came to refer to just the bass and drums. For example, Paul McCartney and Ringo Starr of the Beatles were referred to as the band's rhythm section.
In the 1970s, chordal instruments such as the electric and/or acoustic guitar and various keyboards (piano, electric piano, Hammond organ, clavinet) continued to augment the bass and drums, soul, funk, and reggae groups of the 1960s through 1970s. The sound of late 1960s and 1970s rhythm sections was often given a unique tone and sound due to the use of effects units. Funk bass players would play through auto-wah or envelope follower pedals. Reggae guitarists would plug into echo pedals. Rock guitarists would run their electric guitars through distortion and wah pedals.
In the 1980s, many rock and pop bands continued to be based around the basic rock rhythm section established by 1960s and 1970s bands: electric bass, drums, and electric guitar or keyboards. As electronic effects became more sophisticated during the 1980s, there was some crossover between the roles played by electronic keyboards and electric guitar. Even though electronic keyboards or organs were the standard instruments used to create sustained "pads" of sound (e.g., held backing chords) for ballads, with the introduction of digital delay pedals and other modern effects, electric guitars could produce similar "pads" or "walls of sound". The Edge, the guitarist from the rock band U2, often used delay and reverb-drenched electric guitar arpeggios to create a shimmering, sustained "pad" for the group.
During this era, rhythm sections in some styles of pop took an increasing turn towards electronic instruments. A 1980s-era dance pop band might be backed up by a rhythm section of a synth bass, electronic drums, and various synthesizer keyboards. In some 1980s and 1990s bands, live human rhythm sections were replaced by sequenced MIDI synthesizer rhythm tracks made in the studio. In the 1980s and 1990s, the roots rock scene went in the opposite direction from dance pop; roots rock favoured traditional instruments in the rhythm section such as acoustic piano, acoustic guitar, mandolin, and pedal steel guitar. Another 1980s-era trend that helped revive interest in acoustic instruments was the "MTV unplugged" style of performances, in which a rock band performs with acoustic instruments, including acoustic guitars and an acoustic bass guitar.
In rock and pop, rhythm sections ranged in size from the barest, stripped-down size of the "power trio" (guitarist, bassist, and drummer) and the organ trio (Hammond organist, drummer, and a third instrument) to large rhythm sections with several stringed instrument players (mandolin, acoustic guitar, electric guitar etc.), multiple keyboard players (e.g., piano, Hammond organ, synth), two instruments playing a bass role (e.g., bass guitar and synth bass) and a group of percussionists (congas, shakers, etc.) to fill out the sound.
Musical roles 
The drums and bass both supply a rhythmic pulse for the music, and the bass instrument supplies a harmonic foundation with a bassline. The types of basslines performed by the bass guitarist vary widely from one style of music to another. Despite all of the differences in the styles of bassline, most styles of popular music, the bass guitarist fulfills a similar role: anchoring the harmonic framework (often by emphasizing the roots of the chord progression) and laying down the beat (in collaboration with the drummer). The importance of the bass guitarist and the bass line varies in different styles of music. In some pop styles, such as 1980s-era pop and musical theater, the bass sometimes plays a relatively simple part, and the music forefronts the vocals and melody instruments. In contrast, in reggae or funk, entire songs may be centered around the bass groove, and the bassline is very prominent in the mix.
Similarly, the role of the drummer varies a great deal from one style of music to another. In some types of music, such as traditional 1950s-style country music, the drummer has a rudimentary "timekeeping" role, and the drums are placed low in the mix by the sound engineers. In styles such as progressive rock, Metal, and jazz fusion, the drummers often perform complex, challenging parts, and the drums may be given a prominent placement in the mix; as well, the drummer may be often given prominent solo breaks, fills, or introductions that put the spotlight on their technical skills and musicality. In the more experimental forms of free jazz and jazz fusion, the drummer may not play the strict "timekeeping" role that is associated with drums in pop music. Instead, the drums may be used more to create textured polyrhythmic soundscapes. In this type of situation, the main pulse is often provided by the bass player rather than the drummer.
The rhythm section members sometimes break out of their accompaniment role when they are asked to perform keyboard solos, bass breaks, or drum solos. In genres such as progressive rock, art rock, or progressive metal, the rhythm section members may play complicated parts along with the lead guitar (or vocalist) and perform extended solos. In jazz groups and jazz fusion bands, the rhythm section members are often called on to perform improvised solos. In jazz, the drummer may "trade" short solo sections with a saxophone player or trumpet player; this practice, nicknamed "trading fours", typically involves the drummer and the horn player alternating four bar solo sections during a jazz tune.
Organ trios 
In organ trios, the lower octaves of a Hammond organ or electronic keyboard are used as a substitute for bass guitar or double bass. The organist can play the bassline using the bass pedal keyboard or using the lower manual. As well, the organist could play right-hand chords and melodies. Organ trios were a widely used type of jazz ensemble in the 1950s and 1960s to play hard bop.
Organ trios are sometimes used in rock as well. The Doors' keyboardist Ray Manzarek used a keyboard bass, or bass pedals to play the bass lines. Manzarek, guitarist Robby Krieger, and drummer John Densmore would act as an organ trio with the addition of singer Jim Morrison.
Dixieland bands 
New Orleans or Dixieland jazz bands occasionally use tuba, sousaphone, or bass saxophone in place of the double bass that was common in 1920s-era jazz bands. This tradition developed from the origins of New Orleans music in marching bands, which used instruments that could be carried on harnesses or with straps. Marching bands use a mixture of brass, woodwind, and percussion instruments, because all of these instruments can be played while marching.
Other variants 
Not all rhythm sections follow the standard model of drummer-bassist-chordal instrument. Some bands have no drummer. In bands without a drummer, one or more instruments from the rhythm section often play in styles that replace the drum kit role—that is laying down the beat and backbeat. Traditional bluegrass bands typically do not have a drummer. In bluegrass bands, the timekeeping role is shared between several instruments: the upright bass generally plays the on-beats while the mandolin plays chop chords on the off-beats, with the banjo also keeping a steady eighth note rhythm.
This distributed nature allows for rhythmic continuity while players take turns highlighting the melody. In funk-oriented groups that do not have a drummer, the electric bass player may take over some of the drummer's role by using slap bass. With slap bass, the bassist slaps the low strings to create a strong "thump" (similar to the bass drum's role) and "snaps" or "pops" the high strings to create a percussive effect (the latter takes over some of the role played by the hi-hat cymbals). In some bands, there may be no bass player—the basslines may be played by the piano player, synth player, or guitarist. Using a guitar player to provide basslines is particularly effective if a guitar player has a seven-string guitar with a low "B" string.