16.3 16.2 The ZIP Code

Identification numbers sometimes encode geographic data. The ZIP code, social security numbers, and telephone numbers are the foremost examples. In 1963, the U.S. Postal Service numbered every American post office with a five-digit ZIP code. (ZIP is an acronym for Zone Improvement Plan.) The numbers begin with 0s at the points farthest east—00601 for Adjuntas, Puerto Rico—and work up to 9s at the points farthest west—99950 for Ketchikan, Alaska (see Figure 16.4).

Let's use 55812, one of the ZIP codes for Duluth, Minnesota, as an example.

In 1983, the U.S. Postal Service added four digits to the ZIP code. When four digits are added after a dash—for example, 68588-1234—the number is called the ZIP + 4 code. Mail with ZIP + 4 coding is eligible for cheaper bulk rates, being easier to sort with automated equipment. It's also helpful for businesses that wish to sort the recipients of their mailings by geographic location. The first two numbers of the four-digit suffix represent a delivery sector, which may be several blocks, a group of streets, several office buildings, or a small geographic area. The last two numbers narrow the area further. They might denote one floor of a large office building, a department in a large firm, or a group of post office boxes.

For businesses that receive an enormous volume of mail, the ZIP 1 4 code permits automation of in-house mailroom sorting. For example, the first seven digits of all mail sent to the University of Minnesota Duluth, are 55812-24. The school has designated nine pairs of digits for the last two positions to direct the mail to the appropriate dormitory or apartment building.