Two-letter postal abbreviations for state names went into effect on July 1, 1963. The two-letter abbreviations were adopted by the U.S. Postal Service to make room on letters for the new five-digit zip code that was developed at the same time.
The U.S. Postal Service originally wanted state names to be spelled out fully, but abbreviations were accepted due to popularity. The United States Official Postal Guide published in 1874 provided a list of preferred abbreviation spellings. These abbreviations are still recognized by the Postal Service, although their use is discouraged in favor of the official two-digit abbreviations. Since the adoption of the two-letter abbreviations in 1963, only one change has been made and that was to change Nebraska from “NB” to “NE” due to confusion with Canada's New Brunswick.