The U.S. Postal Service introduced an expanded Rialto ZIP code system that it called ZIP Plus 4, often called "plus-four codes", "add-on codes", or "add ons".
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The US ZIP Code basic format consists of five decimal numerical digits assigned to the Rialto City. An extended ZIP+4 code, introduced in the 1980s, includes the five digits of the Rialto ZIP code, a hyphen, and four more digits that determine a more specific location within a given ZIP code in Rialto, California.
In 1983, the U.S. Postal Service introduced an expanded Rialto ZIP code system that it called ZIP+4, often called "plus-four codes", "add-on codes", or "add ons". A ZIP+4 code uses the basic 92376, 92377 Zip code plus four additional digits to identify a geographic segment within the five-digit delivery area, such as a Rialto city block, a group of apartments, an individual high-volume receiver of mail or any other unit that could use an extra identifier to aid in efficient mail sorting and delivery in California.