All violin bows I have seen are designed to hold a width of at least 1cm of hair. However, in my string training, I was always taught to rotate the bow so that only the outer-most hairs contact the strings. So what is all the other hair for? I imagine the answer is either:
- The extra hair serves only to support the tension of the bow. Or,
- My training is incorrect, and there are times when it is correct to press the bow flat against the strings so they are engaged by the full width of bow hair.
(To elaborate on my training: In my youth I had a decade of classical violin lessons. Revisiting violin technique now I realize that I was always trained to rotate the bow so that only the outer-most hairs (i.e., those opposite my thumb on the bow hand) make contact with the strings. I was taught to modulate bow speed and bow distance from the bridge in order to control volume. The only time it was correct to attack, "pronate," or otherwise press the bow against the strings with more force so that more hair contacted them, was to get the bow to bounce or skip; and in such a case the purpose was to effect a rebound, not to engage the string with more hair. Indeed: intentionally pulling the bow with all hairs flattened across a string seems to only produce a more muddy, scratchy tone.)