V-perforation

V-perforation [De].
A hole made through a piece of bone, stone, or other hard material by grinding or drilling a conical depression from one side only of the material, work proceeding until the tip of the cutter emerges on the other side. See also HOUR-GLASS PERFORATION.
In making buttons, V-perforations involve drilling two convergent holes until they meet at an angle within the body of the button. Especially common in the European early Bronze Age.