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ZIP stands for Zone Improvement Plan, and each of the five digits narrows the map. The first digit marks one of ten areas, from 0 in the Northeast to 9 on the West Coast. The next two digits point to a sectional center facility sorting mail for a region, and the last two name the delivery zone.
Before 1963, mail was sorted mostly by hand, and addresses carried one- or two-digit postal zones in big cities. As mail volume exploded, the Post Office needed machines to do the reading, and machines needed a code with a strict geography built in. The ZIP code, launched in 1963, gave every address in the country a five-digit routing instruction. A cartoon mailman called Mr. ZIP toured the country promoting it, which tells you how much behavior change was involved: the entire nation had to learn to write numbers on envelopes.
The country is divided into ten national areas, numbered 0 through 9, running east to west:
| First digit | Region | Example states |
|---|---|---|
| 0 | Northeast | CT, MA, ME, NH, NJ, RI, VT |
| 1 | DE, NY, PA | |
| 2 | Southeast (upper) | DC, MD, NC, SC, VA, WV |
| 3 | Southeast (lower) | AL, FL, GA, MS, TN |
| 4 | Great Lakes | IN, KY, MI, OH |
| 5 | Upper Midwest | IA, MN, MT, ND, SD, WI |
| 6 | Central | IL, KS, MO, NE |
| 7 | South Central | AR, LA, OK, TX |
| 8 | Mountain | AZ, CO, ID, NM, NV, UT, WY |
| 9 | Pacific | AK, CA, HI, OR, WA |
You can read a lot from one digit. A code starting with 9 is west of the Rockies; a code starting with 0 is in New England or the New York area. The numbering is geographic, not alphabetical, which is why the map reads like a sweep from Maine to Hawaii.
Digits two and three together identify a sectional center facility, or SCF: the large plant that sorts mail for a cluster of post offices around it. The first digit sets the national area, and the next two narrow it to one sorting hub and its territory. That is why neighboring towns often share the first three digits but split on the last two: their mail rides to the same plant, then fans out.
They name the delivery zone: a specific post office, a town, a neighborhood within a city, or in some cases a single large mail recipient. Big organizations that receive enormous volumes can hold their own unique ZIP codes. The IRS, for example, has dedicated codes, because when you get that much mail, you are effectively your own delivery zone.
Not exactly, and this confuses people every year. ZIP codes follow mail routes, which were drawn for delivery efficiency, not for municipal lines. A single ZIP can cover parts of two towns, and one town can contain several ZIPs. Your "city" in a mailing address is really the name of the post office that delivers to you, which is why the town on your mail sometimes differs from the town on your tax bill. The code is about where the truck goes, not where the border is.
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