The Stanley Cup first became the NHL's exclusive championship trophy in 1927. The change followed the collapse of the Western Hockey League after the 1925–26 season, leaving the NHL as the only league competing for the Cup.
Before 1927, the Stanley Cup was a challenge trophy whose champions could come from different Canadian hockey leagues. The Montreal Canadiens, Toronto St. Patricks, Vancouver Millionaires, Seattle Metropolitans, and other clubs competed for it under changing arrangements. The 1927 Final, between the Boston Bruins and Ottawa Senators, was the first all-NHL Stanley Cup Final.
Ottawa won that series in four games, counting one tie under the rules of the era. This history explains why 1927 is more precise than 1926: the 1926 Final was the last Cup Final involving a non-NHL champion, the Victoria Cougars of the Western Hockey League. From 1927 onward, the Cup represented the NHL championship.