The contra-alto flute (also called contrabass flute in G or bass flute in F) is one of the largest instruments in the flute family.

It is in the key of G (or sometimes F), pitched one octave below the alto flute, and a fourth below the bass flute. It is so large that the instrument's body is held vertically, with an adjustable floor peg similar to that of the bass clarinet. (source)