Additionally, with the increasing suburbanization of the U.S., the building of new stadiums and other team facilities in the suburbs instead of the central city became popular from the 1970s. The NFL considers these continuous franchises that relocated to different metropolitan areas. The list also includes franchises from the 1960s American Football League that moved during that league's existence. The NFL and AFL agreed to merge in 1966, with the merger taking effect in 1970. All AFL franchises were accepted into the NFL, and the NFL incorporated the AFL's history, records, and statistics. The NFL considers these separate franchises but there is significant continuity from one to the other    The following are not actually relocations, but temporary moves because these teams' home stadiums were either under construction or otherwise adversely affected:  By the start of the 1920 APFA season, the nascent National Football League was composed of 15 franchises. Of those teams, only two are still in operation (denoted in bold):