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.
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