ST. LOUIS – We get a lot of mail and packages delivered to FOX 2 every day. Even though we’re located in Maryland Heights, well outside St. Louis city limits, our mail is labeled “St. Louis.”

This is a common occurrence with many other cities, municipalities, and unincorporated areas outside St. Louis – places like Clayton, Des Peres, Oakville, University City, and Spanish Lake. Why is that?

The answer lies with the United State Postal Service’s implementation of the Zone Impovement Plan or ZIP Code system.

If you live in the St. Louis area (Missouri, not Illinois), go ahead and check your mail. Your mail starts with a “63” – nearly all the zip codes in eastern Missouri start with the same two digits. It’s that all-important third digit that makes the difference.

If you live in St. Louis City, your zip code will begin with a “631.” That’s the postal designation for the the area. The Kansas City area, for example, has zip codes beginning with a “641.” As the Postal Service explains…

The ZIP Code system is designed to provide an efficient postal distribution and delivery network. ZIP Code assignments are, therefore, closely linked to factors such as mail volume, delivery area size, geographic locations, and topography, but not necessarily to municipal or perceived community boundaries. The general stability of ZIP Code boundaries is essential to prompt and accurate distribution of mail. However, delivery growth and changing demographics can necessitate adjustments to ZIP Code boundaries in order to achieve Postal Service objectives.

USPS Address Management Manual

This Postal Service has dozens of post offices in the St. Louis area (city and county), servicing zip codes ranging from 63101 to 63199. According to the USPS, it’s more efficient for them to serve places like Maplewood and Jennings as “St. Louis.” However, there are some places in St. Louis County that are served with zip zodes starting with “630,” such as Chesterfield, Ballwin, Fenton, and Florissant.

The US Postal Service has a ZIP Code tool that allows you to check for a recommended city name that corrensponds with your zip code.