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United States postage stamps have been demonetized only twice. The first time was in 1851, when the 5-cent and 10-cent stamps of the 1847 issue were declared invalid as of July 1 by the Act of March 3, 1851 reducing the normal letter rate from five to three cents. A few dozen covers are known that carry 1847 stamps after the demonetization date; as stamp usage was then still optional (it would not be made mandatory until 1855), the demonetization seems to have had relatively little impact.

The second, more serious, demonetization was prompted by the outbreak of the American Civil War in 1861. Southern post offices held substantial U.S. assets in the form of their stamp stocks, and the Confederates could theoretically have brought in some income by selling those stamps to private individuals in the North. Although in April 1861 John H. Reagan, postmaster of the CSA, ordered the offices in his charge to return their stamps to Washington D.C., few seem to have done so, and by June 1861 U.S. postmaster-general Montgomery Blair ordered the severance of postal ties and the production of new stamps. In August the stamps of the U.S. 1861 issue began to be distributed throughout the Union, along with orders that postmasters should offer to exchange old stamps for new for a period of six days after giving "public notice through the newspapers and otherwise". After the six-day period was over, that post office was not to recognize the old stamps as paying postage. In addition, postmasters were to accept letters with old stamps from other post offices until set dates, ranging from September 10 in the East, to November 1 from letters arriving from the Far West. (Later the periods were extended for an additional two months.) The process stretched over some months; the large cities in the East were exchanging stamps in the third week of August, while some small remote offices did not start until November. General confusion, combined with exhaustion of the new stamps at some post offices, led to some instances of the old stamps still being accepted on letters after demonetization, although surviving covers are rare.

The U.S. stamps of 1861, and all issued since then, continue to be valid on mail.