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 would one find a four-string double bass with a "B extension", extending the lowest string down to a low B. The extension is an extra section of fingerboard mounted up over the head of the bass. There are several varieties of extensions.