The double bass, also called the string bass, upright bass, bass fiddle, bass violin or contrabass, is the largest and lowest-pitched bowed string instrument in the modern symphony orchestra, with strings usually tuned to E1, A1, D2 and G2 (see standard tuning). The double bass is a standard member of the string section of the symphony orchestra and smaller string ensembles in Western classical music. In addition, it is used in other genres such as jazz, 1950s-style blues and rock and roll, rockabilly/psychobilly, traditional country music, bluegrass, tango and many types of folk music. A person who plays the double bass is usually referred to as a bassist.

The double bass stands around 180 cm (six feet) from scroll to endpin, and is typically constructed from several types of wood, including maple for the back, spruce for the top, and ebony for the fingerboard. It is uncertain whether the instrument is a descendant of the viola da gamba or of the violin, but it is traditionally aligned with the violin family. While the double bass is nearly identical in construction to other violin family instruments, it also embodies features found in the older viol family.