An IOU, abbreviated from "I owe you", is usually an informal document that acknowledges debt. IOUs differ from promissory notes due to the facts that an IOU is not a negotiable instrument and does not specify repayment terms such as the exact time of repayment. These usually specify the debtor, the amount owed, and sometimes the creditor. IOUs may be signed or have distinguishing marks or designs to ensure their authenticity. In some cases, an IOU note may be redeemed for a specific product rather than currency.