Although the U.S. dollar as any other foreign currency is subject to exchange control laws in The Bahamas, the parity between Bahamian dollars and U.S. dollars means that any business will accept either U.S. or Bahamian currency and many of the businesses that serve tourists have extra U.S. dollars on hand for the convenience of American tourists. All banknotes have been undergoing design changes to foil forgery in recent years, although the notes implemented more stringent security long before the U.S.'s recent redesign of their notes. The symbol was later stamped on coins minted in gold and silver. These coins, depicting the Pillars of Hercules over two hemispheres and a small "S"-shaped ribbon around each, were spread throughout America, Europe and Asia. For the sake of simplicity, traders wrote signs that, instead of saying dollar or peso, had this symbol made by hand, and this in turn evolved into a simple S with two vertical bars. Another theory is that it derives from "unit of silver", each unit being one "bit" of the "pieces of eight". Before the American Revolution, prices were often quoted in units of the Spanish dollar.