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. References Philatelic terminology Postal systems