The 19 biggest rule changes in NHL history

The game changed from two 30-minute periods to three 20-minute periods.

Goalies were permitted to fall to the ice to make saves. Previously, a goaltender was penalized for dropping to the ice.

Two lines were painted on the ice 20 feet from center, creating three playing zones, producing a 40-foot neutral center ice area. This is also when assists became an official stat.

A minor penalty would be assessed to any player who delayed the game by passing the puck back into his defensive zone.

One year later, forward-passing rules were instituted at the beginning of the season. This more than doubled the number of goals scored.

A further refinement of the offside rule stated that the puck must first be propelled into the attacking zone before any player of the attacking side can enter that zone. This is the modern offside rule.

A penalty shot was awarded when a player was tripped and thus prevented from having a clear shot on goal, having no player to pass to other than the offending player. Penalty shots at this time were taken from inside a 10-foot circle located 38 feet from the goal. The goaltender was not allowed to advance more than one foot from his goal line when the shot was taken.

Rules were introduced that governed icing the puck.

The penalty shot was modified to allow the puck carrier to skate in before shooting.

Using a zamboni between periods became mandatory.

Because of wartime restrictions on train scheduling, regular-season overtime was discontinued on Nov. 21, 1942.

The red line at center ice was introduced to speed up the game and reduce offside calls. This rule is considered to mark the beginning of the modern era in the NHL. Delayed penalty rules were also introduced at this time.

A player serving a minor penalty was allowed to return to the ice when a goal is scored by the opposing team.

A rule was implemented that prevented players from leaving their benches to enter into an altercation.

A five-minute sudden-death overtime to be played in regular-season games tied at the end of regulation was introduced.

(1) Video replays were employed to assist referees in goal/no goal situations. (2) Major and game misconduct penalties were assessed for checking from behind into boards (boarding). (3) A goal was disallowed if the puck enters the net while a player of the attacking team is standing on the goal crease line, is in the goal crease or places his stick in the goal crease.

High sticking was redefined to allow goals that were scored with a stick below the height of the crossbar of the goal frame.

(1) The crease rule was revised to implement a "no harm, no foul, no video review" standard. (2) Regular-season games tied at the end of three periods, resulted in each team being awarded one point in the standings. As before, there was a five-minute sudden death with four skaters and a goaltender. (3) A team scoring in overtime received one additional point in the standings.

(1) The center line was eliminated for two-line passes, and the tag up offside rule was reinstated. (2) Restrictions on the goaltender playing the puck outside a designated area were introduced. (3) The team icing the puck was disallowed from substituting for the next faceoff. (4) A shootout was introduced if the game remained tied after the five-minute overtime. (5) Goaltender’s equipment was downsized.