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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.
California Registered Warrants 
Also referred to as "IOUs" by the U.S. state of California, the term "Registered Warrants", which specify a future payment date, is meant to differentiate these IOUs from regular, or “normal” payroll warrants which permit the holder to exchange their warrant for cash immediately. For both types of warrants, redeeming them may be delayed until funds are available. Because of this uncertainty, warrants are not negotiable instruments. Registered Warrants were issued in July 2009 due to a temporary inability of the state of California to redeem its warrants. Warrants are issued as payment to state employees, private businesses, local governments, taxpayers receiving tax refunds, and owners of unclaimed money.
In culture 
- IOU: The Debt Threat and Why We Must Defuse It is a book by leading economist Noreena Hertz.
- Guitarist Allan Holdsworth's first studio album was entitled I.O.U.
- "I.O.U." is a chart-topping song by Freeez.
- The Kingdom of Lovely, an Internet-based micronation, uses the "Interdependent Occupational Unit" (abbreviated as IOU) as its currency.
- I.O.U.S.A. is a documentary film about the US national debt.
- In the film Saturday Night Fever, Tony Manero asks Annette if she has an "IOU", but means IUD (intrauterine device).
- "IOU" is a song by the Canadian rock band Metric.
- In the song "Lavender" from Marillion's album Misplaced Childhood, Fish sings "I.O.U. for your love" (written this way in the booklet)
- In the song "Back & Forth" from Foo Fighters' album Wasting Light, Dave Grohl sings "I'm taking it back, all my IOUs", referencing back to a 2009 Foo Fighters song, "Word Forward", featured on the band's Greatest Hits compilation.
- In 2012, episode three of BBC Sherlock, 'The Reichenbach Fall', James Moriarty uses the motif "IOU" as an ambiguous message towards the show's main character Sherlock Holmes.
- The climax of the 1994 movie Dumb and Dumber ends with Lloyd (Jim Carrey) returning a suitcase full of I.O.U's in place of 1 million dollars in cash.
- In the Shining Time Station episode "Schemer Alone", Schemer pays the people who he hired to work for him with nothing but IOUs, which did not go well with them when they found out.
- In the DuckTales episode "Luck o' the Ducks" Scrooge McDuck asks for an IOU when asked to give a gift.
- Daniel X Book 5, Secret branch of the FBI.
- "I.O.U. One Galaxy" is a song from the album End Is Forever by the rockband The Ataris.
- In "The Perfect Foil" episode of ""Murder She Wrote"" an IOU forms a central part of the story line
See also