Understanding the structure of American phone numbers with focus on the first 6 digits: Area Code + Central Office Code (NXX)
The geographic identifier for phone numbers in North America
Originally designed to identify specific regions like New York (212), Los Angeles (213), or Chicago (312).
- First digit: 2-9 (avoids conflict with special services)
- Second digit: 0-9 (cannot match first digit)
- Third digit: 0-9
With mobile phones and number portability, area codes no longer strictly indicate geographic location.
The next 3 digits identifying the local exchange
The Central Office Code (also called exchange code or NXX) follows the area code and consists of:
Originally identified specific telephone exchanges (physical switching centers) serving particular neighborhoods.
How the first 6 digits fit into the complete US phone number structure
443721 Common numbers and their associated regions