Extension (music)

In music, an extension is a set of musical notes that lie outside the standard range or tessitura.

A note that lies outside the lines of a musical staff is an extension of the staff. The note will lie on a ledger line. Middle C for example is an extension note on both treble and bass clefs, however is not outside the grand staff. Soprano C and Deep C lie two ledger lines above treble and below bass respectively (as well as the grand staff).

An instrumental extension is a range of playable notes outside the normal range of the instrument. A baritone horn, if played by a skillful player, can be played an octave above the normal range. Since this isn't standard, these notes would be an extension. See also: Crook (music). With the bowed string instruments, lower pitches than the standard range are sometimes used through scordatura in which the lowest string is tuned down a note or two. The double bass sometimes uses a C extension extending the range of the E string downwards to C. Some Bösendorfer pianos have extra keys, extending the range several notes lower than a standard 88-key piano.

In vocal performance, a singer's extension is all notes that are a part of the singer's vocal range that lie outside the singer's tessitura. This usually include notes that a singer can hit, but doesn't use on a regular basis. For example, a coloratura soprano regularly, as defined by range, will sing in the whistle register. A standard mezzo-soprano has a range to the high F or G above middle C, however a mezzo with good head voice extension can rival the coloratura soprano in range. However, since her normal tessitura is mezzo-soprano (or under Soprano C), her abilities in the whistle register would be considered her extension.

...
Wikipedia