A telephone area code is a three-digit number that helps identify a specific geographic region for phone calls. It is part of the larger telephone numbering plan, which ensures that each phone number is unique. Area codes are used primarily in countries like the United States and Canada, where they help route calls to the correct locations. When dialing a phone number, the area code is usually placed before the local number. For example, in the number (555) 123-4567, "555" is the area code. Area codes can cover large areas or smaller regions, depending on the population density and the number of available phone lines.