In 1910, the National Hockey Association NHA was formed in Montreal. The NHA would further refine the rules dropping the rover position, dividing the game into three 20 minute periods and introducing minor and major penalties. After re organizing as the National Hockey League NHL in 1917, the league expanded into the United States, starting with the Boston Bruins in 1924.
Professional hockey leagues developed later in Europe, since bandy was still popular, but amateur leagues leading to national championships were in place. One of the first was the Swiss National League A, founded in 1916. Today, professional leagues have been introduced in most countries of Europe. Top European leagues include the Kontinental Hockey League, the Czech Extraliga, the Finnish Liiga and the Swedish Hockey League.