How US Area Codes Work

The anatomy of a US phone number

Dial a US number and you're really dialing three parts, written as NPA-NXX-XXXX:

• NPA — the area code (3 digits). "NPA" stands for Numbering Plan Area. It identifies a geographic region, like 212 for Manhattan or 312 for downtown Chicago.
• NXX — the central office prefix (3 digits). Historically this pointed to a specific telephone exchange or switch.
• XXXX — the line number (4 digits). This identifies the individual subscriber line, giving 10,000 numbers per prefix.

The leading "N" in NPA and NXX means that first digit is always 2–9 — never 0 or 1, which the network reserves for signaling. That's why no area code starts with 0 or 1.

What the North American Numbering Plan is

US area codes are part of the North American Numbering Plan (NANP), a shared system created by AT&T and Bell Laboratories in 1947. The NANP covers the United States, Canada, and many Caribbean nations under the single country code +1. The original 1947 plan divided the country into 86 numbering plan areas; states with one telephone region got a code with a 0 in the middle (like 213), while states split into several got a 1 in the middle (like 415). That middle-digit rule ended decades ago, but the codes from that era remain.

Who assigns area codes

Today the numbering plan is administered by the NANPA (North American Numbering Plan Administrator) under the oversight of the FCC and state public utility commissions. When a region is projected to exhaust its supply of prefixes, the administrator works with the state regulator to introduce relief — either a split or an overlay.

Why new area codes keep appearing

Each area code holds a finite pool of numbers. Mobile phones, second lines, fax machines, data SIMs, and number portability all consume the supply faster than population growth alone would suggest. When a region nears exhaustion, a new code is added. Decades ago this meant splitting the map; since the early 2000s almost all relief is done with overlays, because they let everyone keep their existing number. See the newest area codes for what's activating now.