If someone owes money, write an IOU, including the title, text and signature. The specific contents need to include the legal full names of the debtor and the lender, the specific amount of the loan, the repayment method and date, the loan interest rate, etc.

IOUs are usually documentary evidence written by civil subjects when they owe money or things to others. The IOU reflects the confirmation of creditor's rights and debts by both parties, but it can't explain the reasons of creditor's rights and debts.

The validity of IOUs refers to the statute of limitations of IOUs, which does not affect the validity of IOUs. As long as it is a legally signed, authentic and valid IOU, the IOU itself is valid no matter how long it takes.