The standard length of time by which the length of an NHL game is measured. Every NHL game, whether it is a preseason, regular season or playoff game, consists of three twenty-minute periods.
Even though an NHL period is twenty minutes long, it takes longer than twenty minutes in real time to complete an NHL period. Therefore, a period does not consist of twenty minutes of continuous play because the clock counting down the time remaining in a period will be stopped every time there is a stoppage in play (eg.: a goal, a penalty, an off side, an icing, etc.).
For preseason and regular season games, if the score is still tied after the three twenty-minute periods are completed, a additional five-minute period will be played. If the score remains tied, then the winner of the game will be decided by a shootout. However, in playoff games, if the score is tied after the three twenty-minute periods, additional twenty-minute periods will be played until a goal is scored to decide the game's winner.