""JB perhaps somebody could invent a capo for the violin. then cut little notches on the neck for positions/keys""

There's already a perfectly adapted capo for the violin, which can in fact do far more than the standard bar type capos commonly used on guitars.

It is a rather more complex five piece setup which can be very rapidly moved up and down the neck, to achieve the desired effect.

It is normally referred to as the "Hand".

Don T.