The trombone is unique among brass instruments because it uses a telescopic slide to change the length of the tubing (and thus the pitch) instead of valves. This allows the trombone to play every microtone between notes, a feature called a 'glissando.' Historically, the trombone (originally known as the sackbut) was associated with religious music and was often used in cathedrals to double the vocal lines of church choirs. Because of its powerful, resonant sound, it was sometimes referred to as the 'Voice of God.' It wasn't until Beethoven’s Fifth Symphony that the trombone became a permanent member of the symphony orchestra.