It took an ad campaign to sell Americans on its value

Mr. ZIP, a cartoon mail character created in the early 1960s to entice Americans to use ZIP codes.

1963 was a momentous year in America: President John F. Kennedy was assassinated, Martin Luther King Jr. delivered his “I Have a Dream” speech at the March on Washington and, somewhat less heralded by all but the most fervent postal historians, the ZIP code was introduced.

Since its founding in 1775, the post office relied on hand sorting based on local addresses to get mail where it was supposed to go. A piece of mail often went through 10 postal workers before making it to its recipient. But by the 1940s, the then-named Post Office Department realized its sorting system was not keeping pace with the growing population of the country it served.

In 1943, as a way to streamline mail sorting for the biggest American cities, the post office began placing one and two digit numbers between the city and the state to help clerks wade through the increasing volume of mail, which was then around 20 million pieces per year.

(MORE: How to Fix the Post Office: Keep the ‘Last Mile,’ Outsource the Rest)

By the early 1960s, the post-war population boom and continued western growth led to even greater use of the postal service. Mail volume doubled between 1943 and 1962, putting further pressure on the post office to sort mail efficiently. On July 1, 1963, on the recommendation of an internal advisory board, the post office introduced the Zone Improvement Plan Code, which divided the entire country into coded delivery areas. The first two or three numbers told carriers to which states mail was being sent. More populous regions like New York were given five digit numbers starting with 10-14, for example, whereas less populous areas like Montana received five-digit numbers. These new ZIP codes helped the post office better pinpoint where mail was headed while allowing it to expand machine-based sorting systems that could quickly read digits. But many Americans were reluctant to adopt the new system.

“People were concerned they were being turned into numbers,” says Jennifer Lynch, a U.S. Postal Service historian. “They thought it was depersonalizing them.”

To get people on board, the post office began an extensive marketing campaign centered around Mr. ZIP, a friendly looking cartoon mail carrier. A folk group called The Swingin’ Six sang about ZIP Code usage in a lengthy public service announcement video. “Put ZIP in your mail” ran in magazines across the country, including TIME, while a series of short TV ads showed postal workers drowning in a sea of letters and used slogans like “Only you can put ZIP in your postal system.”

Evidently the ads were persuasive. In 1966, three years after ZIP Codes were introduced, 50% of Americans said they used ZIP Codes. By 1969, 83% said they did, according to a 1969 study conducted by Roper Research Associates.

In 1983, the post office expanded the ZIP Code to nine digits to identify which side of the street the mail was being delivered to, as well as particular office buildings. Today, ZIP codes are translated into “automation-readable barcodes” that are placed on pieces of mail when sorted and contain 31 digits of information that tell the post office everything from whether it was presorted, if the mail is first-class or a periodical, and even which business sent it. It also allows the U.S.P.S. to track virtually every letter and package around the country.

(MORE: U.S. Postal Service to Launch Clothing Line)

The post office estimates that increased efficiencies for both large mailers and the postal service itself add almost $10 billion of value to the U.S. economy a year.

Today, 50 years after the ZIP code debuted, the postal service’s Office of the Inspector General is recommending that they be linked to digital geographic information systems based on latitude and longitude to further increase delivery accuracy. This time, however, Mr. ZIP will stay in retirement.

MORE: Why Los Angeles Is a Bad City to Be a Letter Carrier