The World Series is the annual championship series of Major League Baseball (MLB), a professional baseball league operating in the United States and Canada, contested between the champion teams of the American League (AL) and the National League (NL). The winning team, determined through a best-of-seven playoff, is declared the champions of the league and awarded the Commissioner's Trophy. It was first played in 1903 to determine the championship team of baseball after the American League declared itself the second major-league circuit in the United States to compete with the previously established National League.