If you live in the City of Pittsburgh, your mailing address is obvious: Pittsburgh, PA. But some residents in neighboring boroughs and townships can also use a Pittsburgh mailing address when they’re sending a package or a postcard.
Carnegie Mellon University technical writer and South Hills resident Matt Mroz was curious about why someone living outside of Pittsburgh’s limits could receive mail with the city’s name.
“I always wondered about how some of the zip codes in our metro area are actually very far from Pittsburgh,” Mroz asked. “So something like Plum is considered Pittsburgh, Pennsylvania.”
Listener Mike from Aspinwall had a similar query: “I live with my family in Aspinwall, but if I’m filling out an address form, I can list my city and ZIP as either Pittsburgh, PA 15215, or Aspinwall, PA 15215, and it will work.”
Both listeners are correct in that Aspinwall and Plum are boroughs outside of the City of Pittsburgh limits. But they’re still connected to the city through their development and historically established postal routes. At the core of those routes is the ZIP code, a multi-digit system used by the United States Postal Service (USPS) to ensure birthday gifts, mail-in ballots and yes, even bills, efficiently arrive in your mailbox.
The ZIP code’s origins
We can trace the origins of the USPS to the Second Continental Congress in 1775. The new, young United States was beginning to grow and needed a formal agency to sort and distribute dispatches. In 1792, it was officially named The Post Office Department. (It wasn’t renamed the United States Postal Service until 1971.) As the country spread, so did the postal service, developing routes through big cities and rural towns and sorted manually by workers.
Jump forward to the 1940s, and the Post Office Department realized the manual sorting system they’d relied on couldn’t keep up with the booming population. They needed a way to process mail more quickly, and wanted to start automating some of their operations. That automation would need a way to read the packages and letters coming through the system, so they adopted a standardized two-digit coding system that had been used in some major cities beginning in 1943.
The two-digit coding system worked well enough that in 1963, the post office moved to a more elaborate five-digit ZIP code. Each of those five digits play an important role in making sure mail ends up where it should, according to USPS strategic communications specialist Mark Lawrence.
Let’s take the Aspinwall example of 15215. The very first number, or “1” in this case, represents a broader multi-state region including Pennsylvania, New York and Delaware. New England states start with “0”; numbers increase as you move west across the United States — for example, Illinois ZIP codes start with 4, Colorado ZIPs start with 8, and the West Coast (along with Hawaii and Alaska) all begin with 9.
The second and third digits identify the regional postal network; in Aspinwall’s case, “52” indicates Pittsburgh. The “52” regional sorting facility is located in the city’s California-Kirkbride neighborhood, and serves a chunk of the immediate metro area.
In addition to all of Pittsburgh, the 152 ZIP codes extend out to Upper Saint Clair in the South, McCandless in the North and Penn Hills in the East. The Pittsburgh network and sorting facility also handle 153 and 154 ZIP codes, which include Washington County and parts of the Mon Valley.
If you go just a bit further north, to New Castle, you get a “160 to 162” sorting code; out east, you’ll find “156” in Greensburg. All these areas have their own regional network and associated sorting facility.
The final two digits in the zip code, Lawrence said, are the specific, physical post office, likely near where you live. This is the last stop for your mail before your postal carrier brings it to your mailbox. For Mike in Aspinwall, 15215 is the post office located at 1200 Main St. in Sharpsburg.
In the 1980s, four additional numbers were added to the zip code to further assist USPS’ sorting machines, but they aren’t necessary to include.
Oh the places you’ll go
When it comes to places like Aspinwall and Plum, the reason that you can write “Pittsburgh, PA” on your letter is the fact that the zip code sorting system was built upon historical mail routes.
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Lawrence said it’s important to remember that this system is not designed to reflect community borders, but rather help the post office easily and efficiently deliver mail. And the city or town name comes second to the zip code. “We will recognize other names,” Lawrence said. “If somebody were to put their township on there, and the proper ZIP code, it should get there.”
And it’s possible that your designated post office might not even be the closest one to your house, which you can also chalk up to those historic mail routes. ”Even though [someone] could have a post office two miles away from their house per se, and there's another one three miles away, their mail might be delivered out of the post office that’s three miles away because we already have all those routes established,” said Lawrence.
There are 41,683 zip codes across the country. In this city, the University of Pittsburgh, Duquesne University and some hospital systems have their own ZIP code. But some of the area’s most prominent features don’t get one.
“The rivers wouldn't have a ZIP code,” Lawrence said. “I mean, if there's an island in the middle of a river that has a business on it or a residence on it, hey, they would need a delivery address and they would have a ZIP code.”
Fun fact: To encourage people to use the ZIP code system in 1962, USPS created a cartoon character named “Mr. ZIP,” or “Zippy.” The jovial fast-walking mail carrier character was designed by advertiser Howard Wilcox in hopes that young people would adopt the ZIP code in their letter-writing practices and teach older generations how to, as well.
