Guides to ZIP codes and area codes

How the two numbering systems work, why they don't match, and what they mean when you choose or keep a phone number. Each guide links the sources for its facts.

• How area codes and overlays workWhat an area code is, how a new one is added when an old one fills up, and what an overlay means for a town: two or more codes in one place, old numbers kept.
• Why ZIP codes and area codes don't line upZIP codes follow mail delivery routes and area codes follow telephone rate centers, so their edges rarely match. Why that is, and how this site links the two.
• Ten-digit dialing: why local calls need the area codeWhy many local calls now need the area code: overlays, and the 988 crisis line change of 2021 and 2022. What seven-digit, ten-digit and 1+10 dialing each mean.
• ZIP codes, ZCTAs, P.O. boxes and ZIP+4How ZIP codes work, how the Census Bureau maps them as ZCTAs, why some ZIP codes have no area on a map, and what the four extra digits of ZIP+4 are for.
• Getting a local number in another cityHow a business can have a local number in a city where it has customers but no office: what the area code says, how calls reach you and which code to pick.
• Keeping your phone number when you moveHow to keep a phone number when you move or change phone service: how a transfer starts, why not to cancel first, how long it takes, and what fees can apply.