The octobass is an extremely large and rare bowed string instrument that was first built around 1850 in Paris by the French luthier Jean-Baptiste Vuillaume (1798–1875). It has three strings and is essentially a larger version of the double bass - the specimen in the collection of the Musée de la Musique in Paris measures 3.48 metres (11 ft 5 in) in length, whereas a full-size double bass is generally approximately 2 m (6 ft 7 in) in length.

Because of the extreme fingerboard length and string thickness, the musician plays it using a system of levers and pedals. The levers serve to engage metal clamps that are positioned above the neck at specific positions. These clamps act as fretting devices when the performer presses the levers.

The only known work from the Nineteenth Century that specifically calls for the Octobass is Charles Gounod's Messe solennelle de Sainte-Cécile. In this work, the Octobass only appears in the "Benedictus" and the "Agnus Dei." It typically plays one octave below the Double Bass.

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