These standards define scratches and digs according to their physical size and mark their grade with the letters: A, B, C, D, E, F, G (and H which is used only for digs). The letter A represents the narrowest scratch, which is 0.005 mm wide and the smallest dig, which is 0.05 mm in diameter. On the other hand, the letter G represents a scratch that is 0.12 mm wide and a dig that is 0.7 mm in diameter. A microscope or magnifying glass is used for testing, or sometimes even just using the naked eye to compare to a master.