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This first-ever ZIP Code was 00601, designating a decent chunk of Puerto Rico as the first Zone Improvement Plan (ZIP) in the United States.

ZIP is an acronym for Zone Improvement Plan. However, the USPS intentionally chose the acronym to indicate that mail travels more quickly when senders mark the postal code on their packages and envelopes.

The lowest and highest of these postal codes are both specific to areas in Alaska; The 00001 code is for the N Dillingham area, and the postal code 99950 is for Ketchikan, AK.

The term ZIP is an acronym for Zone Improvement Plan; it was chosen to suggest that the mail travels more efficiently and quickly (zipping along) when senders use the code in the postal address. The term ZIP Code was originally registered as a service mark by the USPS; its registration expired in 1997.

The first number of the five-digit code signifies the region which the address is located in, a number that grows from the east coast to the west. For example, Eastern states such as Maine and New York begin with 0 or 1, whereas the Western states of California and Washington begin with a 9.

In the early days of the U.S. Postal Service, mailing addresses weren't regulated. You might have used the recipient's street address along with the city and state, but you wouldn't have seen a ZIP code. Mail was hand-sorted, and delivery relied on local knowledge of its intended destination.

Standard US ZIP codes The standard ZIP code notation used by the United States Postal service uses five digits to identify a delivery area. An example of a standard US ZIP code is 90210.