The sackbut is a Medieval and Renaissance brass instrument that uses a telescopic, U-shaped slide tube to alter pitch. It looks very much like the modern trombone but could be found in various sizes with the slides sometimes going backward instead of forward. The sackbuts more conical and less dramatic flare of the bell as well as its quieter sound is what differentiates it from the modern trombone.

More Info: en.wikipedia.org