History
The National Hockey League expanded to ten teams for the 1926-27 season and split into two divisions, the American and Canadian. The league was now in sole possession of the Stanley Cup, but reluctant to give up on the post-season championship series, because it was so popular with fans.
From minor-league baseball, the league borrowed the idea of an extensive post-season playoff period. This was intended in part to keep fans interested in the regular season competition for playoff spots.
The playoffs have gone through a bewildering variety of formats since 1927. They'll be explained here only in broad strokes.
Through the 1937-38 season, the NHL used playoff systems that featured play between the two divisions to determine cup finalists, with various combinations of two-game, total-goal, three-game, and five-game series.
Oddly, the idea of seeding was pretty much ignored throughout this period. In the first round, for example, the two first-place teams played one another, so one of them was always eliminated before the semi-finals.
The NHL shrank to seven teams in 1938-39, and six of them got into the playoffs. When the league shrank again, back to the "original six" in 1942-43, only four teams made the playoffs.
In 1967-68, the league doubled in size to 12 teams. The existing were grouped into the East Division, while all six expansion teams went into the new West Division. All the playoff rounds leading up to the finals were kept within the divisions, thus ensuring that one of the expansion would reach the final round.
Six more teams were added in 1974-75, when the NHL went to a four-division format, with three teams from each division moving into the playoffs and the regular-season division champions receiving first-round byes.
The next round of expansion came in 1979, when the NHL absorbed four teams from the defunct World Hockey Association. The number of teams advancing to the playoffs was increased from 12 to 16, with the four division winners and the 12 other teams with the best regular-season records.
Currently, the NHL is split into two conferences, each of which has six divisions. Eight teams from each conference go into the playoffs, with the three division champions as the top seeds and the other five teams selected and seeded based on their regular-season records.
