Bills of exchange are negotiable instruments that contain an unconditional order to pay a sum of money. They must be in writing, signed by the maker, contain an order to pay money only, specify the amount to be paid, and fix a time of payment. The key parties are the drawer, who issues the order; the drawee, on whom it is drawn; and the payee, to whom the money is to be paid. Acceptance by the drawee makes them liable to pay the bill on the date of maturity. Bills can be discounted, endorsed, renewed or retired, and dishonored bills must be noted and protested. Related books include bills receivable and bills payable.