The busiest time of year for the US Postal Service coincides with the December holiday season, when we’re all busy mailing greetings and gifts alike. But USPS workers are busy year-round: postal employees process a whopping 181.9 million of pieces of first-class mail per day!

The 470,000 employees who work for the USPS (including the 7,000 on the Fleet of Feet delivering mail on foot) rely heavily on the five- or nine-digit ZIP codes for efficient and reliable mail delivery. But have you ever thought about what ZIP codes actually are? Beyond the cultural cachet of knowing (or even living in) “90210,” we don’t typically memorize them beyond our home addresses, so we tend to overlook the importance of those five digits.

What does ZIP mean?

ZIP is an acronym for Zone Improvement Plan. However, the USPS intentionally chose the acronym to indicate that mail travels more quickly when senders mark the postal code on their packages and envelopes. It makes sense that the term ZIP code would be related to being zippy, which means “lively” or “peppy.”

The general system of ZIP codes used today was implemented in 1963. Prior to this system, the USPS used a system of postal zones that was only applicable to large cities. This is where the “improvement” part comes into play.

The basic ZIP code has five digits. The first three digits refer to a sectional center facility (or SCF), what is basically a network of super post offices. All of the post offices that have those three digits in their ZIP code have their mail sorted and processed by the same SCF. The last two numbers designate the specific post office within an SCF’s territory.

What is a ZIP+4?

In the 1980s a new system was introduced called ZIP+4. Four additional digits (preceded by a hyphen) were added to the basic code. This allowed senders to indicate an even more precise location, such as a particular block or apartment building. The last two numbers, in fact, designate which segment or side of the street. The rise in post office boxes also made this greater level of precision necessary.

What about something that actually goes in the mail? Learn about absentee vs. mail-in ballots here!