ZIP Codes 101

[molito]

The first three digits of a ZIP Code together usually indicate the sectional center facility to which that ZIP Code belongs. This facility is the mail sorting and distribution center for a zone or area. Some sectional center facilities have multiple three-digit codes assigned to them. For example, the Northern Virginia sectional center facility in Merrifield is assigned ZIP Codes beginning in 220, 221, 222, and 223.

The fourth and fifth digits of the ZIP Code represent the area of the city or town. For example, if a letter is received with a ZIP Code of 47722, the USPS can know that it's in Indiana (4), it's in Vanderburgh county (77), and it's in the area of the University of Evansville (22).

ZIP Codes are Lines, Not Shapes

ZIP Codes are not drawn according to state boundaries. In fact, since they are designed only to increase mailing efficiency, ZIP Codes can and do cross county and state boundaries. For example:

• 42223 spans Christian County, Kentucky, and Montgomery County, Tennessee
• 97635 includes portions of Lake County, Oregon, and Modoc County, California
• 51023 and 51001 are in both Iowa and Nebraska

Although most ZIP Codes can be roughly assigned to a geographic area, ZIP Codes do not represent geographic regions; ZIP Codes denote address groups or delivery routes. Because of this, ZIP Code "areas" can overlap, be contained within another ZIP Code, or have no geographic area. For example ZIP Codes that start with 095 are assigned to the Navy, and therefore have no geographic location. Similarly, areas without regular postal routes (rural route areas) or areas with no mail delivery (undeveloped areas), are not assigned ZIP Codes. They are based on sparse delivery routes, so the boundary between ZIP Code areas there is indefinite.