Area Code History
How North American area codes evolved to meet growing demand
When the North American Numbering Plan was designed in the 1940s, area codes were assigned based on population and calling volume. As populations grew and businesses required more phone lines, the original codes began running out of available numbers. Two solutions emerged: splits and overlays.
Area Code Splits
A geographic split divides an area code's territory: part of the region keeps the original code while the remainder is reassigned a new one. Residents in the new region must update their area code.
Area Code Overlays
An overlay assigns a new area code to the exact same geographic region as an existing code. Both codes coexist in the same area, requiring 10-digit dialing for all local calls.
Why Do Area Codes Get Split or Overlaid?
Each area code supports approximately 8 million possible phone numbers (800 exchanges × 10,000 numbers each). As population density increases, demand for new phone numbers — from residents, businesses, fax machines, pagers, and later cell phones — can exhaust an area code's supply.
Splits were historically the preferred solution: a new boundary line is drawn, and one half of the territory gets a new code. The downside is that residents and businesses in the affected area must update their printed materials, business cards, and advertising.
Overlays became more common starting in the 1990s because they don't require anyone to change their existing number. However, they do require all callers in the area to dial 10 digits even for local calls.