Most modern Boehm system bass clarinets have an "extension" key allowing them to play to the (written) E♭. This key was originally added to allow easy transposition of parts for the relatively rare bass clarinet pitched in A, but it now finds significant use in concert band and other literature. A significant difference between soprano and bass clarinet key work is a key pad played by the left-hand index finger with a vent that may be uncovered for certain high notes. This allows a form of "half-hole" fingering that allows notes in higher registers to be played on the instrument. In addition, older bass clarinets have two register keys, one for middle D♯ and below, the other for middle E and higher. Newer models typically only have one. The second register key makes the altissimo range much easier to play. Many professional and advanced bass clarinetists own instruments with extensions down to a C (sounding B♭ identical to the bassoon's lowest B♭, two octaves below written middle C. At concert pitch this note is the B♭ below the second ledger line below the bass staff or B♭1 in scientific pitch notation. Overall, the instrument sounds an octave lower than the B♭ soprano clarinet.