A fan letter from New Zealand and addressed simply to Alfred E. Neuman, mascot of MAD magazine, once found its way to the publication’s New York City offices. There was no address or ZIP code printed: The fan had simply drawn a picture of Neuman’s gap-toothed image on the envelope.

For everyone else, there are ZIP codes. But why do mail carriers like UPS and USPS require those five-digit codes on correspondence? Why do we use them?

According to Reader’s Digest, it started with a postal inspector named Robert Moon. In 1943, Moon had the idea for a postal zone system that could better sort mail given the staffing shortages created by World War II and increasing reliance on air mail as opposed to the hand-sorting that took place on rail cars. Moon’s suggestion was to identify the entire country using a numerical code.

The first number would represent a group of states, from zero on the east coast to nine on the west coast. The second and third numbers identified the specific mail sorting facility the letter would go to. Later, the USPS added two more numbers to track which post office handled the mail.

Moon, who was described as the “undisputed father of the first three digits in the code” by The New York Times, clearly had a sound idea—but it was initially met with resistance owing to politics. According to his wife, Moon was a Republican but the government was Democrat-controlled at the time. It took years and repeated attempts for his idea to be seriously considered, she said.

Building on Moon’s initial idea, the five-digit Zone Improvement Plan (ZIP) code went into effect on July 1, 1963. People weren’t necessarily enthusiastic about this more efficient system, with some complaining that it was yet another number to remember in addition to their social security and telephone numbers. But the grumbling eventually ceased, and the ZIPs simply became routine.

In 1983, the USPS added a four-digit addendum that further drilled down mail locations to specific apartment buildings or streets.

Sometimes, a location may be deemed important or singular enough to deserve its own ZIP designation. The White House, for example, has its own ZIP (20006). So do some major universities.

ZIPs are also highly coveted in the retail industry, where cashiers often solicit a customer’s ZIP in order to find out more personal information—like a mailing address to send flyers—using a name and postal code. ZIPs are also sometimes used to confirm a person’s identity before using a credit card, the theory being that a thief wouldn’t know it offhand.

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