The Campeonato Paulista de Futebol Profissional da Primeira Divisão - Série A1, simply known as the Campeonato Paulista, is the top-flight professional football league in the Brazilian state of São Paulo. The league is contested between 20 clubs and typically lasts from January to April. Rivalries amongst four of the most well-known Brazilian teams (Corinthians, Palmeiras, Santos and São Paulo) has marked the history of the competition. The "Paulistão", as the tournament is popularly known, is also the oldest established league in Brazil. It was first held in 1902.