U.S. Patent 3,525,284 (August 25, 1970): METHOD AND APPARATUS FOR FILING FINGERNAILS OF GUITARISTS, BANJO PLAYERS.
An abrading device comprises a flat sheet of abrasive material such as sandpaper, affixed to a backing support member having opposite edges folded over to form grasping means. The abrading device is placed over the strings of an instrument such as guitar. The musician simulates his playing of the instrument, instead of plucking the strings, running his fingernails over the surface of the abrasive material thereby filing the fingernails.
In subsequent actual playing, the edges of the fingernails are thus file do that they are substantially parallel to the strings when such strings are plucked, thereby assuring optimum plucking action.
I wonder if we can get a patent for fingernail filing for the harp. Better yet - for keyboard players who play software instruments. We can include a nail file with every copy of GPO!
And musicians, be careful! Filing your fingernails a certain way may lead to patent infringement!