A modern orchestral double bass usually has four strings.
The four strings are commonly tuned in fourths, from low E to A, D and G. This tuning differs from the fifth-based tuning used by violin, viola and cello, reflecting the double bass’s historical and practical role as the lowest standard bowed string instrument in the orchestra.
Some double basses have a fifth string, especially instruments designed for extended orchestral range or particular performance traditions. Many orchestral four-string basses also use an extension on the lowest string so that it can sound pitches below the normal low E.
The double bass has a complex history involving viol family and violin-family features, so its exact ancestry is not a simple straight line. It is also called the contrabass or upright bass. In jazz and popular music, it may be played pizzicato rather than with a bow, while orchestral players use both techniques.