An extensive minor league baseball system covers most mid-sized cities in the United States. Minor league baseball teams are organized in a six-tier hierarchy, in which the highest teams (AAA) are in major cities that do not have a major league team but often have a major team in another sport, and each level occupies progressively smaller cities. The lowest levels of professional baseball serve primarily as development systems for the sport's most inexperienced prospects, with the absolute bottom, the rookie leagues, occupying the major league squads' spring training complexes.