The contrabass clarinet and contra-alto clarinet are the two largest members of the clarinet family that are in common usage. Modern contrabass clarinets are pitched in BB♭, sounding two octaves lower than the common B♭ soprano clarinet and one octave lower than the B♭ bass clarinet. Some contrabass clarinet models have a range extending down to low (written) E♭, while others can play down to low D or further to low C. Some early instruments were pitched in C; Arnold Schoenberg's Fünf Orchesterstücke specifies a contrabass clarinet in A, but there is no evidence of such an instrument ever having existed. (source)