ZIP (Zoning Improvement Plan) codes were introduced on July 1, 1963, in order to allow the U.S. postal system to sort mail more quickly. ZIP codes were also a part of the system's switch to faster, more automated equipment.
When you send a letter with a five-digit ZIP code on it to a U.S. address, it first is sent to the correct state, and then the first three digits of the ZIP code are read (by an electronic or manual reader) to determine which "hub" postal office the letter is to go to. The last two digits of the ZIP code determine what mail carrier route the letter is to take to get to the correct address. The ZIP code can also have four additional numbers, in the form of "-1234", which determine the specific street and house. It's better to use as many numbers as possible in a ZIP code, because it improves accuracy and may be faster. It is possible to send a letter with simply nine digits on it, such as "12345-6789".
Your local post office has a directory of nine-digit ZIP codes, and it's the size of a good-sized phone book. This stack contains most of the ZIP codes in the U.S., and should do fine for general use.
About The ZIP Codes In This Stack
This stack shows the five-digit ZIP codes to places in the U.S. or related territories. For many locations, there is often more than one ZIP code. This means that every ZIP code in that place has the same first three digits, but does not have the same last two digits in every section of that place. It is possible to have a nine-digit ZIP codes listing on disk, but since the unabridged listing on computer media costs hundreds of dollars to buy from the U.S. Postal Service I can't provide it, sorry!
