Highly effective and inexpensive tool for detecting bad bills and deterring counterfeiters from passing phony currency in the first place Simply make a small mark on U.S. currency. Chemically sensitive ink works instantly. An amber mark passes the test, while a dark brown or black mark means the bill is probably counterfeit. Works on all new design U.S. currency (bills series 1959 and later). Pen-style for extreme portability.