Area codes were developed in 1947 to meet the post-World War II surge in demand for telephone services. The first area code to be used was New Jersey's 201 area code in 1951..
In recent years, the demand for more telephone numbers has exhausted the supply of telephone numbers that are available in certain geographic areas and led to the need for additional area codes. Many new telephone numbers are needed for fax machines, cellular phones, pagers, computer lines and second lines. By adding new area codes, millions of new numbers have become available to accommodate the demand.
Area codes are assigned according to the North American Numbering Plan (NANP). It was invented in 1947 by AT&T and Bell Laboratories. It conforms to the International Telecommunications Union Recommendation E.164, which is the international standard for numbering plans.
The NANP is the numbering plan for the Public Switched Telephone Network in the United States and its territories, Canada, Bermuda, and many Caribbean nations. NANP numbers are ten-digits in length in the following format: NXX-NXX-XXXX, where N is any digit from 2 to 9 and X is any digit from 0 to 9. The first three digits are usually called the area code. The second three digits are called the central office code, or prefix. The final four digits are the line number.