String instruments require players to select pitches by depressing strings at specific intervals. On an instrument like a guitar, mandolin, banjo, or electric bass, this is known as “fretting.” On a fretless instrument like a violin, viola, cello, or double bass, this is known as “stopping” the string along the fingerboard.
Violin is one of the more difficult instruments to produce accurate pitches on. This is due to two factors: the fretless nature of the instrument; and the fact that violins produce high-pitched notes which are harder to play in tune than lower-pitched notes. (Learn more about notes and octaves here.)
In order to reliably play notes in tune, violinists use a series of hand positions to make sure their fingers are aligned with the correct places on the fingerboard.