The double bass (sometimes called the contra-bass or acoustic bass) is the lowest pitched member of the orchestral strings family. The strings family consists of the violin, the viola, the cello, and this, the double bass. It is about 6 ft in height,  and it has four strings. The lowest string is tuned to the third E below middle C,  (E1 using MIDI note-naming) and the next three strings ascend in fourths.  So the second string is the 3rd A below middle C, the third string is the second D below middle C, and the top string is the second G below middle C (MIDI G2).  To make the score easier to read, lots of ledger lines are avoided by printing the double bass part one octave higher than the true pitch.

In the orchestra, this instrument is played with the bow, or plucked for a pizzicato sound. It is sometimes called the contrabass. In jazz and popular groups, it is always plucked, and it is often called the "Acoustic Bass", to distinguish it from the electric bass guitar. The instrument rests on the ground, and  the player either stands up, or sits on a special high stool.

In the General MIDI system, the plucked double bass is called the "Acoustic Bass", and is midi instrument number 32, whereas the bowed instrument is called the "Contrabass" and is MIDI instrument number 43.

Strictly speaking, it is not a true member of the violin family, but a descendant of the ancient viol family. This is made apparent by the sloping neck, whereas the violin, viola and cello all have rounded necks.

Since the pitch is so deep, very little music has ever been written for the double bass as a solo instrument.  It normally plays the lowest notes in any orchestration, often sounding the root notes of chords, laying the foundation for the harmony.

The most famous exception is the solo passage for the double bass called "The Elephant", from Saint-Saens's Carnival of the Animals.