History of organizational changes in the NHL
Since being founded in 1917, the National Hockey League (NHL), which in its first two seasons started out as a three-team league and eventually grew to thirty in its current state, has expanded and contracted numerous times throughout its history. The following is a complete history of organizational changes in the NHL.
Early years 
Three teams (1917–19) 
The four teams that began the inaugural NHL season were the Montreal Canadiens, the Montreal Wanderers, the original Ottawa Senators, and the Toronto Arenas. However, after completing four games out of the scheduled 22, the Wanderers withdrew from the league due to their arena burning down, and the NHL continued this season and the next with only three teams.
Four teams (1919–24) 
In its third season, 1919–20, the NHL underwent its first expansion, adding the Quebec Bulldogs. Toronto changed its name to Toronto St. Patricks. The next season, however, Quebec relocated to Hamilton, becoming the Hamilton Tigers. These same four teams continued playing for four seasons, up to 1923–24.
Six teams (1924–25) 
Seven teams (1925–26) 
Ten teams (1926–31) 
The NHL continued to expand the following season, adding the Chicago Black Hawks, the Detroit Cougars, and the New York Rangers, growing to ten teams, thus more than doubling its size in its first decade of existence. During the 1926–27 season, Toronto was renamed the Toronto Maple Leafs, taking effect next season. For the 1930–31 season, the Pirates moved from Pittsburgh to Philadelphia, becoming the Philadelphia Quakers, and Detroit was renamed the Detroit Falcons.
Eight teams (1931–32) 
Nine teams (1932–35) 
For the 1932–33 season, after missing one season, the original Ottawa Senators rejoined the NHL, and the Detroit Falcons were renamed the Detroit Red Wings. Two seasons later, for the 1934–35 season, the Ottawa Senators relocated, becoming the St. Louis Eagles.
Eight teams (1935–38) 
The Eagles folded after one season, and the NHL was once again an eight-team league for three seasons.
Seven teams (1938–42) 
The Montreal Maroons withdrew from the league for the 1938–39 season, further reducing the number of teams in the NHL to seven, shrinking to the size the league was in 1925–26. Play continued for four seasons with seven teams, with the New York Americans changing their name to the Brooklyn Americans for the 1941–42 season, their last.
Original Six and expansion years 
Six teams (1942–67) 
The 1942–43 season saw the folding of the Brooklyn Americans, thus ushering in the Original Six era of the NHL, which lasted without any organizational changes for twenty-five seasons until the 1967 expansion, which doubled the number of teams in the league.
Twelve teams (1967–70) 
For the 1967–68 season, six new teams were added to the NHL: the California Seals, the Los Angeles Kings, the Minnesota North Stars, the Philadelphia Flyers, the Pittsburgh Penguins, and the St. Louis Blues. During their first season, the California Seals were renamed the Oakland Seals.
Fourteen teams (1970–72) 
Sixteen teams (1972–74) 
Eighteen teams (1974–78) 
Once again after two seasons, two more teams started play in the NHL, the Washington Capitals and the Kansas City Scouts. Two seasons later, however, the California Golden Seals relocated and became the Cleveland Barons, and the Kansas City Scouts moved as well, becoming the Colorado Rockies.
Seventeen teams (1978–79) 
Twenty-one teams (1979–91) 
Four teams joined the NHL the next season, coming over from the defunct rival league, the World Hockey Association (WHA). These were the Edmonton Oilers, the Hartford Whalers, the Quebec Nordiques, and the Winnipeg Jets. This doubled the number of Canadian teams in the league. The following season, Atlanta relocated and became the Calgary Flames. In turn, for the 1982–83 season, the Colorado Rockies moved, becoming the New Jersey Devils. Chicago changed the spelling of their name from the Black Hawks to the Blackhawks for the 1986–87 season, based on its original franchise documents. Despite these relatively minor organizational changes, this was one of the more stable periods of NHL history, lasting twelve seasons.
Further expansion 
Twenty-two teams (1991–92) 
Twenty-four teams (1992–93) 
Twenty-six teams (1993–98) 
The next season, another two teams were added, the Florida Panthers and the Mighty Ducks of Anaheim. The Minnesota North Stars relocated, becoming the Dallas Stars. Two seasons later, for the 1995–96 season, the Quebec Nordiques relocated and became the Colorado Avalanche. The following season, the Winnipeg Jets also moved, becoming the Phoenix Coyotes. The season after that, the Hartford Whalers relocated, becoming the Carolina Hurricanes.
Twenty-seven teams (1998–99) 
Twenty-eight teams (1999–2000) 
The following season, another team started play, the Atlanta Thrashers.
Thirty teams (Since 2000) 
The NHL added the Columbus Blue Jackets and the Minnesota Wild in its 84th season, reaching the current roster of 30 teams. The Mighty Ducks of Anaheim changed their name to the Anaheim Ducks in the 2006–07 season. The Atlanta Thrashers moved to Winnipeg, Manitoba, becoming the new Winnipeg Jets in the 2011–12 season.
Possible expansion 
Despite recent statements from the NHL that no further expansion or even relocation is planned for the foreseeable future (a statement partially contradicted by the relocation of the Thrashers to Winnipeg), there have been rumors and talks of potential new sites for existing or new teams in various locations in the United States and Canada. Since Winnipeg (the previous frontrunner) received the Jets in 2011, Quebec City has been the most-discussed potential relocation site; other potential markets for relocation that have seen action in recent years include Saskatoon, Seattle, Kansas City, and Southern Ontario (although the league has actively blocked all of the Southern Ontario efforts to date, citing territorial concerns with the Buffalo Sabres and Toronto Maple Leafs).