A rhythm section (also called a backup band) is a group of musicians within a music ensemble or band who provide the underlying rhythm and pulse of the accompaniment, providing a rhythmic reference and "beat" for the rest of the band. Many of the rhythm section instruments, such as keyboard instruments and guitars, are also used to play the chord progression upon which the song is based. The bass instrument (either double bass or electric bass, or another low-register instrument, such as synth bass, depending on the group and its style of music) plays the low-pitched bassline that supports the chord progression, typically by emphasizing the roots of the chords (and to a lesser degree the fifths and thirds of the chords).