An IOU (abbreviated from the phrase "I owe you") is usually an informal document acknowledging debt. An IOU differs from a promissory note in that an IOU is not a negotiable instrument and does not specify repayment terms such as the time of repayment. IOUs usually specify the debtor, the amount owed, and sometimes the creditor. IOUs may be signed or carry distinguishing marks or designs to ensure authenticity. In some cases, IOUs may be redeemable for a specific product or service rather than a quantity of currency, constituting a form of scrip.