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Is there a law or regulation that governs the maximum allowable interest amount that can be charged on credit cards or in agreements where credit is extended?
In the US, usury is complicated and depends on the type of account, the bank charter and the where the bank makes credit decisions. Most major US credit cards are issued by entities in Utah, South Dakota and Delaware. None of these states have usury limits. Many states have usury limits. In New York, for example a loan may not exceed 16% interest, if the institution is supervised by the State.  Credit card issuers are usually chartered as "National Associations" (ie Federally chartered banks regulated by the Comptroller of the Currency). There is no Federal usury statute, and Federally chartered banks are allowed to "export" many of the regulations of the state where credit decisions are made. Small states like South Dakota basically design their banking regulations to meet the needs of the banks, which are major employers.