An area code is a 3-digit sequence of numbers to group together all the telephone numbers in a region. When the system was implemented back in 1951, each state had one or two area codes. Now, the state of California alone has 20 area codes. New codes are constantly being put into service as existing codes run out of new numbers to assign.

Area codes are part of an overall North American Numbering Plan that defines the format of telephone numbers in the US, Canada, US territories and possessions, and most of the English-speaking Caribbean countries. Under this plan, a telephone number is 10 digits long and consists of:

• a 3-digit area code, or Numbering Plan Area (NPA)
• a 3-digit prefix, or exchange
• a 4-digit line number

Due to a rule forbidding prefixes to begin with 0 or 1 and another rule reserving the N11 prefixes (ex. 911) for special uses, there are 792 available prefixes in each area code.