The bass trombone is the bass instrument in the trombone family of brass instruments. Modern instruments are pitched in the same B♭ as the tenor trombone but with a larger bore, bell and mouthpiece to facilitate low register playing, and usually two valves to fill in the missing range immediately above the pedal tones. The earliest bass trombones were the bass sackbuts, usually pitched in G, F, or E♭ below the B♭ tenor. They had a smaller bore and less flared bell than modern instruments, and a longer slide with an attached handle to allow slide positions otherwise beyond the reach of a fully outstretched arm.
The modern bass trombone has evolved from the large-bore B♭/F tenor-bass trombones in the late 19th century. In the early 20th century, manufacturers attempted to solve the problem of the missing low B♮1 on such instruments by adding a second valve. In the 1920s, manufacturers Conn and Holton made B♭/F bass trombones with a Stellventil (lit. 'static valve') that could lower the F tubing to E when manually set. The first true double-valve trombone (where the second valve can be operated while playing) was made by Olds in 1937, using a second dependent valve to lower the instrument to E, and having no effect alone.
Since the Romantic period, the trombone section of an orchestra, wind ensemble, or British-style brass band usually consists of two tenor trombones and at least one bass trombone. In a modern jazz big band, at least one of the trombonists will play bass trombone, often serving as the anchor of the trombone section or doubling the double bass and baritone saxophone.