Mr. Zip, a gangly cartoonish figure with wide friendly eyes and a neat blue mail carrier's uniform, emerged fifty years ago to help the U.S. Postal Service promote its newest idea: five numbers added to our addresses to more clearly designate our locations. In 1963, the post office was overwhelmed with billions of pieces of mail each year, and suburban sprawl was spreading Americans farther and farther away from each other. At most post offices, people still sorted mail by hand, putting letters one by one into pigeonholes. The best employees could sort faster than one piece of mail per second, but it wasn't enough. What was needed was machine sorting. And machines read numbers, not handwritten addresses.

"Put ZIP in your mail!" exclaimed a cheery promotional poster. Another ad, featuring a certain yellow-hatted detective, read: "Dick Tracy says: 'Protect your mail! Use ZIP codes!'" But the number that began as a sorting utility has since expanded far beyond our addresses. Today, our ZIP code determines how we are read by policy-makers, politicians, statisticians, pollsters, insurers, businesses, organizers, and marketers. Governments use ZIP codes to determine who gets what—and this, in turn, stokes our political divisions. Private companies use ZIP code information to determine if they will, or will not, move into our communities. Retailers collect ZIP codes from customers, which can protect against fraud, but also helps a consumer database marketer collect personal information on us without our permission.

ZIP codes, in other words, have evolved from finding where we are to defining who we are—far beyond our mailbox. "Organizations—business, government—can look at the mass of people we've become and break us down into usable points," says Nancy Pope, curator at the Smithsonian's National Postal Museum. "While it was designed to help our letters travel faster, it's become like an ID system we all agree to and all use."

As ubiquitous as they are now, the public originally balked at Zone Improvement Plan (ZIP) codes, even though they were (and still are) voluntary. ZIP codes came on the heels of telephone area codes, and Social Security numbers. People were wary of having another number to learn and use. Humorist Art Buchwald, writing a month after ZIP codes debuted, penned a syndicated column complaining about "the numbers racket":

The purpose of all these numbers, it is carefully explained to the American people, is to help speed up the American way of life. The truth of the matter is that while you do the work for the companies by playing their numbers game, they can lay off thousands of workers and use computers instead.

ZIP codes seemed to forbode a future where individual identity mattered less and less. Unlike area codes or Social Security numbers, ZIP codes narrowly locked in your location for use by what was then a department of the president's cabinet. It made some uneasy. It's why the likes of Ethel Merman were called on to help with the enormous promotional campaign: She performed in a public service announcement singing about ZIP codes to the tune of "Zip-a-Dee-Doo-Dah." The song was even released as a 45.