The ZIP code is the five digit number at the end of a United States mailing address that helps direct your letter to where it needs to go. But how is it possible to indicate every place in such a large country using only 5 numbers? The secret is in the way the codes are assigned—they aren’t just given out one to each city in numerical order. The first number in the ZIP represents a specific cluster of several states. In general, the numbers are lower to the north and east, and higher in the south and west. Thus, all the ZIP codes in Maine start with 0, while California gets a 9 for theirs. The second and third numbers indicate a Sectional Center Facility within those regions. These are not your ordinary post offices—they are not open to the public, and they typically operate overnight so that they can get the mail sorted and sent out by the next day. A Sectional Center Facility is responsible for handling mail from nearby cities, each of which is indicated by the final two numbers of the ZIP. The assigning of those numbers is not entirely consistent, but for the most part they begin with the largest city in the area, and after that are distributed alphabetically by city name. Large cities may have more than one ZIP code, and some mail-intensive addresses, such as universities or corporate offices, may have their own. But even so, there are fewer than 99 ZIPs handled by each Sectional Center Facility, which means that even as the country’s population continues to grow, the mail will still be able to ZIP comfortably along.