What does extension mean in music?

Left hand & fingerboard

Extension

An extension is a temporary widening of the left-hand frame, most commonly on cello, that reaches a note a half step outside the position without shifting the whole hand.

On the cello, the normal first-position frame spans a minor third between the first and fourth fingers. A forward extension moves the first finger back a half step, and a backward extension moves the second, third, and fourth fingers up, widening the span to a major third. The thumb travels with the extension to keep the hand from twisting.

Extensions let a cellist play scale passages that would otherwise require a shift in the middle of a slur. They are a cello-specific solution to a cello-specific problem: the violin's frame already covers a perfect fourth, and the bass's string is too long for the technique to help.

Position: A position is a fixed placement of the left hand on the fingerboard, numbered by which note the first finger falls on, so that a set of pitches is reachable without moving the hand.
Shifting: Shifting is moving the left hand to a new position on the fingerboard, and it is the technique that gives a string player access to the instrument's full range.
Simandl fingerings: Simandl fingerings are the standard orchestral double bass system, using the first, second, and fourth fingers in the lower positions because the hand spans only a whole step there.
Intonation: Intonation is accuracy of pitch, on a fretless string instrument, the ongoing skill of placing each finger so the note is in tune with the harmony around it.

Every term in this glossary is taught in context somewhere in Arco's curricula, on the staff, on the keyboard, and on your own fingerboard at the same time.

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