The United States Post Office Department (USPOD) implemented postal zones for large cities in 1943. For example:

The "16" is the number of the postal zone within the city.

By the early 1960s a more general system was needed, and on July 1, 1963, non-mandatory ZIP codes were announced for the whole country. Robert Moon, an employee of the post office, is considered the father of the ZIP code; he submitted his proposal in 1944 while working as a postal inspector.[citation needed] The post office gives credit to Moon for only the first three digits of the ZIP code, which describe the sectional center facility (SCF) or "sec center." An SCF is a central mail processing facility with those three digits. The SCF sorts mail to all post offices with those first three digits in their ZIP codes. The mail is sorted according to the final two digits of the ZIP code and sent to the corresponding post offices in the early morning. Sectional centers do not deliver mail and are not open to the public (though the building may include a post office open to the public), and most of the workers are employed to work night shift. Mail picked up at post offices is sent to their own SCF in the afternoon, where the mail is sorted overnight. In the cases of large cities, the last two digits coincided with the older postal zone number,[citation needed] thus:

In 1967, these were made mandatory for second- and third-class bulk mailers, and the system was soon adopted generally. The United States Post Office used a cartoon character, Mr. ZIP, to promote use of the ZIP code. He was often depicted with a legend such as "USE ZIP CODE" in the selvage of panes of stamps or on labels contained in, or the covers of, booklet panes of stamps. Curiously enough, the only time the Postal Service issued a stamp promoting the ZIP code, in 1974, Mr. ZIP was not depicted.[citation needed]

In 1983, the U.S. Postal Service began using an expanded ZIP code system called "ZIP+4", often called "plus-four codes", "add-on codes" or "add ons."

A ZIP+4 code uses the basic five-digit code plus four additional digits to identify a geographic segment within the five-digit delivery area, such as a 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. Initial attempts to promote universal use of the new format met with public resistance,[citation needed] and today the plus-four code is not required. In general, mail is read by a multiline optical character reader (MLOCR) that almost instantly determines the correct ZIP+4 code from the address and—along with the even more specific delivery point—sprays a Postnet barcode on the face of the mailpiece that corresponds to 9 digits.

For post-office boxes, the general (but not invariable) rule is that each box has its own ZIP+4 code. The add-on code is often one of the following: the last four digits of the box number (e.g., PO Box 58001, Washington DC 20037-8001), zero plus the last three digits of the box number (e.g., PO Box 12344, Chicago IL 60612-0344), or, if the box number consists of fewer than four digits, enough zeros prepended to the front of the box number to produce a four-digit number (e.g., PO Box 52, Garrett Park MD 20896-0052). However, there is no uniform rule, so the ZIP+4 code must be looked up individually for each box.

It is common to use add-on code 9998 for mail addressed to the postmaster (to which requests for pictorial cancellations are usually addressed), 9999 for general delivery and other high-numbered add-on codes for business reply mail.[citation needed] For a unique ZIP code (explained below), the add-on code is typically 0001.

The ZIP code is often translated into a barcode called Postnet that is printed on the mailpiece to make it easier for automated machines to sort. Unlike most barcode symbologies, Postnet uses long and short bars, not thin and thick bars. The barcode can be printed by the person who sends the mail (some word-processing programs such as WordPerfect and Microsoft Word include the feature), or the post office will put one on when it processes the piece. The post office generally uses OCR technology, though a human may have to read the address if absolutely necessary.

People who send bulk mail can get a discount on postage if they have printed the barcode themselves. This requires more than just a simple font; mailing lists must be standardized with up-to-date Coding Accuracy Support System (CASS)-certified software that adds and verifies a full, correct ZIP+4 code and an additional two digits representing the exact delivery point.[citation needed] Furthermore, mail must be sorted in a specific scheme and be accompanied by documentation verifying this. These steps are usually done with PAVE-certified software that also prints the barcoded address labels and barcoded sack or tray tags.