In the United States , Canada and United Kingdom , most professional orchestral players use four-string double basses with a `` C extension , '' which extends the lowest string down as far as low C , an octave below the lowest note on the cello . In the United States , Canada and United Kingdom , most professional orchestral players use four-string double basses with a `` C extension '' , which extends the lowest string down as far as low C , an octave below the lowest note on the cello ( more rarely , this string may be tuned to a low B ) .