The double bass, or simply the bass, is the largest and lowest-pitched bowed string instrument in the modern symphony orchestra. It is a transposing instrument, which means it sounds an octave lower than written.
The double bass is common for string orchestras, as well as concert bands, chamber music ensembles. Particularly, since the 1950s with the popularity of jazz music, the double bass gained itself a prestige roll in modern music, becoming an indispensable member in styles such as blues, rock, country, bluegrass, tango, and folk.
The bass plays an integral role in any ensemble, whether it be a rock band, jazz band or symphony orchestra. In terms of chordal movement, bass players follow in the grand tradition of counterpoint and voice-leading epitomized by Johann Sebastian Bach. Chords which are made up of two or more notes played simultaneously, and a bass player has the choice to trigger any one of them at any one time. As a harmonic instrument triggering some of the lowest frequencies within any musical entity, the bass lays the tonal foundation for how music flows from moment to moment.
Playing the bass will inevitably make you a better composer/arranger. Some of the best modern composer/arrangers are also bass players such as John Clayton, Randy Jackson, Victor Wooten, Paul McCartney, Sting, and Charles Mingus to name a few. A solid knowledge of music theory and harmony gives them their expertise. That foundation is rooted in the bass. If you devote some time to learning how the bass functions in a given musical composition, you will develop a crucial understanding of how to craft arrangements of your own. This knowledge will also aid you greatly if you choose to composer music.