Another term for a double bass, acoustic bass, or bass fiddle. It's the largest of the string instruments (resembling a giant violin) and lowest-sounding instrument found in an orchestra. Can be played with a bow or plucked with the fingers. The word "upright" means that it is to be played while standing up while holding the instrument upright, as opposed to how a bass guitar is held (on the lap or at a 45 degree angle). Also not to be confused with the cello, which is smaller and played while sitting down.
