An area code is a section of a telephone number which denotes the broad area that the phone receiving the call is based in. It is the section just before the local number, and just after both the access and country codes. This section usually doesn’t need to be dialed if the number being called is in the same area as the number making the call, unlike the local number, which must always be dialed in its entirety.
In the United States, an area code is a three digit number that comes before the seven digits that make up the local number, three for the prefix and four for the suffix. While the prefix of the local number gives an idea of the more specific area, such as town or neighborhood, the area code denotes the larger region, either a whole segment of a city, or even an entire county of part of a state.
The system used by the United States is part of a larger system, known as the North American Numbering Plan (NANP). The NANP is a standard used by twenty-four countries: the United States, Canada, US territories, and a number of Caribbean countries. The NANP sets out basic rules to ensure easy interoperability when it comes to phone numbers between these countries, including limitations on the formats these codes can have.
The NANP lays out rules for how these codes can be formatted, effectively limiting the number of area codes that are possible. The form followed allows for the first digit to be any number between two and nine, the second digit may be any number between zero and eight, and the third digit may be any number between zero and nine.
Since area codes originated at a time when phones were rotary, rather than digital, the codes themselves were assigned based on theoretical usage. This meant that areas with high populations would receive codes that would dial more quickly on a rotary phone, while areas with low populations could receive longer ones. For example, Los Angeles was given 213, Chicago 312, New York City 212, and Detroit 313. This means that New York City required only five clicks to dial the area code, and Detroit would require only seven. In contrast, Vermont was given 802, North Carolina was given 704, South Carolina 803, and South Dakota 605, each requiring twenty or twenty-one clicks.
Originally, the middle digit of the code was also meant to give some information about the area code distribution within the state. The middle digit originally could be only a zero or a one, as a way to help switching equipment know that the number being dialed was not the prefix for a local number. A code with a middle digit of zero was understood to be used for the entire state, while one with a one was understood to be covering a subsection of the state. As phone use increased drastically, the system changed, and the number of area codes open for use increased, although certain restrictions remained, such as not using a zero, reserved for the operator, or one, reserved for the country code, as the first digit.