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

Staff

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 grandstaff. Soprano C and Deep C lie two ledger lines above treble and below bass respectively (as well as the grand staff).

Instruments

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.

Voice

In vocal performance, a singer's extension is all notes that are a part of the singer's vocal range that lie outside the vocal range of the singer's fach. This usually include notes that a singer can hit, but don't use on any regular basis. For example, a coloratura soprano regularly, as defined by range, will sing in the whistle register. A mezzo-soprano however, may be able to access the whistle register as well. However since the her normal tessitura is mezzo-soprano (or under Soprano C), her abilities in the whistle register would be considered her extension.

Note that for a singer who possesses a full voice, the extension would be located underneath the lower fach and above the higher fach. A man with that can sing bass and tenor for example, would have an extension below F2 and above C5 (Tenor C).

For men, their vocal extension is usually co-terminus with his falsetto, unless they are a countertenor, barbershop tenor, or its equivalent in popular music.