This is the list of original North American Numbering Plan area codes of 86 plan areas as defined by AT&T in 1947. In preparation for direct distance dialing, AT&T and the Bell System developed the North American Numbering Plan in the 1940s. The plan divided the United States and Canada into numbering plan areas (NPAs) and assigned a three-digit dialing prefix to each. Over the course of the decade following introduction of these routing codes, local subscriber numbers were standardized to seven digits. This included a three-digit central office prefix, dialed as the first two letters of the local central office name and one digit, and the four-digit subscriber station number.