The US Post Office periodically adds new ZIP codes in response to increased mail volume. There are at least 40,000 five-digit ZIP code areas in the US. The ZIP code was developed in the early 1960s to facilitate speed of mail handling. The postal code system began on July 1, 1963, and every address in the US had an assigned ZIP code by that date. The zip+4 system was introduced in 1978 to expedite processing of mail.
The number of ZIP codes in the United States changes periodically as the US Post Office finds it necessary to add new codes in response to increased mail volume. According to the US Census Bureau and other sources, there are at least 40,000 five-digit ZIP code areas in the United States In a one-year period from November 2006 to 2007, approximately 200 ZIP codes were added in the United States
The ZIP code, an acronym for Zoning Improvement Plan, became necessary in the early 1960s as business mail increased to such an extent that it accounted for nearly 80% of mail traffic. An advisory committee to the USPS department has recommended the development of a coding system to facilitate speed of mail handling by increasing the use of mail sorting machinery. The particular system, the postal code system, began on July 1, 1963.
Postal zones, already in place since the early 1940s, helped develop the new postcodes. The new system required realignment of the entire mail system. The Metro System envisaged the installation of mail transport centers in 85 cities in the United States. Over 550 sectional centers have since been developed. Finally, the particular codes were assigned to each center and therefore the particular addresses served by each.
By the start date of July 1, 1963, every address in the United States had an assigned zip code. The first figure referred to one of the ten geographical areas of the USA; the next two digits indicated a town. The last two digits divided population centers into small postal centers or zones within larger cities. Initially, no one was required to use postcodes.
In 1967, the USPO required those shipping items second- and third-class mail to pre-order with the five-digit code. In 1978, the postal system found it necessary to add a dash and four more digits to postal codes to expedite processing of the ever-increasing volume of mail. The resulting zip+4 system was in effect in 1983. The additional four digits allowed mail to be sorted by block or by a particular office building. Like its predecessor, the five-digit postcode, zip+4 has reduced the number of times a piece of mail has to be handled.
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