North American area codes are part of the North American Numbering Plan (NANP), a unified telephone numbering system used by 20+ countries including the United States, Canada, and several Caribbean nations. Each area code is a three-digit prefix that identifies a specific geographic region, helping route calls efficiently across cities and states. First introduced in 1947, area codes have expanded over time due to population growth and mobile phone usage, with many cities now using multiple overlapping codes.