Although area codes began with a sort of pattern, with the rapid increase of phone number usage, that pattern has since been abandoned. As of 2015, area codes are handed out to areas on an as-needed basis.
Area codes were originally assigned by AT&T. The original pattern behind assigning area codes was that larger cities that used more phone numbers received lower numbers. New York, Los Angeles and Philadelphia, for example, all have area codes beginning with 2. In addition, in some states, phone companies assigned area codes based on the first letter of the city. However, as telephone usage has skyrocketed, this pattern was abandoned, and area codes are now assigned as they are needed without any regard to the number itself.