A stablecoin is a token issued by a private company that promises to be worth one dollar. The company keeps that promise by holding dollars and short-dated United States Treasury bills, which makes the largest issuers standing lenders to the American government. In July 2025 Congress gave them a federal charter, the GENIUS Act, which fixes what they may hold, who may issue them, and what the issuer must be able to do with a token when a court or an agency orders it. The Stablecoin sets out, from the statute and the published data, how private digital money came to finance part of the federal debt and why the Treasury welcomes it; what the Act changes for banks, savers and the wider economy; how a holder is taxed, and what he forgoes in interest; and what the Act's compliance provisions mean in practice, set beside the account closures reported from Vietnam and Thailand and the cashless program in Dubai. Each case is given as the authority stated it: the legal basis, the date, the number as the source gives it, and what happened to the money. The book then asks whether any of this is new. The National Bank Acts of 1863 and 1864 created private banks that issued dollar notes backed by government bonds deposited with the Comptroller of the Currency, so that a wartime Treasury gained a buyer of its debt while the public got a uniform currency. The free banking era before it, the bank holiday of 1933, the money market funds of 2008 and the deposit conversions in Cyprus in 2013 each answer the same two questions: under what law, and what happened to the money afterwards. The strongest case for the Act is given fairly before the book answers it. What the law permits is separated from what a future government might do with it, and the second is labeled as speculation. Every statutory statement is footnoted to the section of the Act, and every number to its source. The book reports the record and gives no financial advice. Felix Nikolas Prehn is an economist and former investment banker.