At the first tournament in 1920, there were many differences from the modern game: games were played outdoors on natural ice, forward passes were not allowed, the rink was 56x18 metres (the current International standard is 61x30 metres) and two twenty-minute periods were played.
Each side had seven players on the ice, the extra position being the rover.
Following the tournament, the IIHF held a congress and decided to adopt the "Canadian rules"–six men per side and three periods of play.

At an IIHF congress in 1969, officials voted to allow body-checking in all three zones in a rink similar to the NHL.
Prior to that, body-checking was only allowed in the defending zone in international hockey.
The IIHF later described the rule change as "arguably the most substantial and dramatic rule changes in the history of international hockey" because it allowed for a more aggressive game.
The rule, which was first applied at the 1970 World Championships, was controversial: IIHF president Bunny Ahearne feared it would make ice hockey "a sport for goons."
Several other rule changes were implemented in the early 1970s: players were required to wear helmets starting in 1970 and goaltender masks became mandatory in 1972.
In 1992, the IIHF switched to using a playoff system to determine medalists and decided that tie games in the medal round would be decided in a shootout.
The IIHF decided to test a new rule in 1997 that would allow two-line passes.
Prior to that, the neutral zone trap had slowed the game down and reduced scoring.
At the 1997 World Championships, teams were allowed to decide if they wanted to test the rule.
Although no team accepted the offer, the rule was adopted.
The IIHF described it as "the most revolutionary rule change since allowing body-checking in all three zones in 1969. [...]
The new rule almost immediately changed the game for the better.
The 1999 IIHF World Championship in Norway was a stark contrast to the finals the year before with many more goals scored and with end-to-end action – not defence – dominating play."

The current IIHF rules differ slightly from the rules used in the NHL.
One difference between NHL and IIHF rules is rink dimensions: the NHL rink is narrower, measuring 61x26 metres (200x85 feet), instead of the international size of 61x30.5metres (200x100feet).
Another rule difference between the NHL and the IIHF rules concerns how icings are called.
As of the 2013–14 regular NHL season, a linesman stops play due to icing using the hybrid icing method, instead of the former method, where a defending player (other than the goaltender) touched the puck before an attacking player was able to, in contrast to the IIHF rules that use "no-touch" icing, where play is stopped the moment the puck crosses the goal line.
The NHL and IIHF differ also in penalty rules.
The NHL, in addition to the minor and double minor penalties called in IIHF games, calls major penalties which are more dangerous infractions of the rules, such as fighting, and have a duration of five minutes.
This is in contrast to the IIHF rule, in which players who fight are ejected from the game.

Since the 2005–06 season, the NHL instituted several new rules.
Some of them were already used by the IIHF, such as the shootout and making the two-line pass legal.
Others which were not picked up by the IIHF, such as requiring smaller goaltender equipment and the addition of the goaltender trapezoid to the rink.
However, the IIHF did agree to follow the NHL's league's zero-tolerance policy on obstruction and required referees to call more hooking, holding and interference penalties.
In 2006, the IIHF voted to eliminate tie games and institute a three-point system: wins in regulation time would be worth three points, overtime wins would be two points and over-time losses would be worth one point.
The system was first used at the 2007 World Championships.

Since 2019, the World Championships banned the shootout for the Gold Medal Game.
Multiple 20-minute golden goal overtime periods of 3-on-3 are played until whoever scores, which wins the game.
At the first tournament in 1920, there were many differences from the modern game: games were played outdoors on natural ice, forward passes were not allowed, the rink was 56x18 metres (the current International standard is 61x30 metres) and two twenty-minute periods were played. Each side had seven players on the ice, the extra position being the rover. Following the tournament, the IIHF held a congress and decided to adopt the "Canadian rules"–six men per side and three periods of play.
At an IIHF congress in 1969, officials voted to allow body-checking in all three zones in a rink similar to the NHL. Prior to that, body-checking was only allowed in the defending zone in international hockey. The IIHF later described the rule change as "arguably the most substantial and dramatic rule changes in the history of international hockey" because it allowed for a more aggressive game. The rule, which was first applied at the 1970 World Championships, was controversial: IIHF president Bunny Ahearne feared it would make ice hockey "a sport for goons." Several other rule changes were implemented in the early 1970s: players were required to wear helmets starting in 1970 and goaltender masks became mandatory in 1972. In 1992, the IIHF switched to using a playoff system to determine medalists and decided that tie games in the medal round would be decided in a shootout. The IIHF decided to test a new rule in 1997 that would allow two-line passes. Prior to that, the neutral zone trap had slowed the game down and reduced scoring. At the 1997 World Championships, teams were allowed to decide if they wanted to test the rule. Although no team accepted the offer, the rule was adopted. The IIHF described it as "the most revolutionary rule change since allowing body-checking in all three zones in 1969. [...] The new rule almost immediately changed the game for the better. The 1999 IIHF World Championship in Norway was a stark contrast to the finals the year before with many more goals scored and with end-to-end action – not defence – dominating play."
The current IIHF rules differ slightly from the rules used in the NHL. One difference between NHL and IIHF rules is rink dimensions: the NHL rink is narrower, measuring 61x26 metres (200x85 feet), instead of the international size of 61x30.5metres (200x100feet). Another rule difference between the NHL and the IIHF rules concerns how icings are called. As of the 2013–14 regular NHL season, a linesman stops play due to icing using the hybrid icing method, instead of the former method, where a defending player (other than the goaltender) touched the puck before an attacking player was able to, in contrast to the IIHF rules that use "no-touch" icing, where play is stopped the moment the puck crosses the goal line. The NHL and IIHF differ also in penalty rules. The NHL, in addition to the minor and double minor penalties called in IIHF games, calls major penalties which are more dangerous infractions of the rules, such as fighting, and have a duration of five minutes. This is in contrast to the IIHF rule, in which players who fight are ejected from the game.
Since the 2005–06 season, the NHL instituted several new rules. Some of them were already used by the IIHF, such as the shootout and making the two-line pass legal. Others which were not picked up by the IIHF, such as requiring smaller goaltender equipment and the addition of the goaltender trapezoid to the rink. However, the IIHF did agree to follow the NHL's league's zero-tolerance policy on obstruction and required referees to call more hooking, holding and interference penalties. In 2006, the IIHF voted to eliminate tie games and institute a three-point system: wins in regulation time would be worth three points, overtime wins would be two points and over-time losses would be worth one point. The system was first used at the 2007 World Championships.
Since 2019, the World Championships banned the shootout for the Gold Medal Game. Multiple 20-minute golden goal overtime periods of 3-on-3 are played until whoever scores, which wins the game.
Ice Hockey World Championships