String players have both the benefit and curse of being able to play more than one note at a time. This technique is known as double-stopping (i.e. putting fingers down (stopping) two strings at once, and bowing the two strings simultaneously). Unlike other instruments with strings (the piano or guitar for example), we don’t have either fixed pitches played by keys, or frets on our fingerboards. So there is much gnashing to teeth and rending of garments when we are forced to fix intonation on two notes that sound at the same time. It can take on a surprising amount of complexity.
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