An area code is a three-digit number that identifies the geographic area where a phone number is assigned. The United States, Canada and specific Caribbean countries have area codes.
As phones grew in popularity during the 1940s, the nation needed a way to help route calls. AT&T developed the first area codes that went into effect in 1947. At that time, area codes were known as the North American Numbering Plan and included all of North America, including Canada.
The first area codes had a zero as the middle number and in 1947, there were 86 area codes in use. Eventually, the population grew and new area codes had to be created. It was during the early 1950s that area codes using a 1 in the middle were implemented. Many of these area codes, such as 212 for New York City, are still in use as of 2015.
When area codes were first introduced, people used rotary phones. Telephone companies believed in making area codes easier on people by using low numbers to reduce dialing time. Once touch tone phones became more popular, larger numbers were used as area codes.
The first three digits of a phone number, after the area code is called the office code or the central office code. The last four digits of a phone number are called the subscriber number or directory number.