When can a fourth substitute be used at the World Cup 2018 and have they ever been used before in the competition?
The big tournament has reached the knockout stages and that means it could be time for the fourth sub
FOR the first time in World Cup history, teams involved in matches that go to extra-time will be allowed to use a fourth substitute.
This is the most significant change to World Cup substitution procedure since fresh legs from the bench were first allowed in the 1970 tournament.
When can a fourth substitute be used in the World Cup?
The option to use a fourth substitute in a World Cup match will become available to managers in matches that go to extra-time.
The dramatic potential for extra-time as a way of settling a match comes into effect in a match that is level after 90 minutes in the knockout stages of the tournament.
Has a fourth sub been used in a World Cup before?
This year will be the first time that managers can introduce a fourth set of fresh legs in a World Cup match that goes to extra-time.
But it is not the first time that a manager has been granted this option in other tournaments.
It has been used several times including in the 2017 women's European under-17s tournament, women's Euros and under-19 Championship, as well as the 2016 FIFA Club World Cup and Olympic Games.
A fourth sub will also be used in the UEFA Champions and Europa leagues' knockout rounds from next season.
Alvaro Morata became the first fourth substitution in men's football when he was brought on for Cristiano Ronaldo in extra-time at the 2016 Club World Cup.
Kelechi Iheanacho replaced Raheem Sterling in the 2017 FA Cup semi-final defeat against Arsenal to make history as the first fourth sub to be used in England.
A fourth substitute was used for the first time at the World Cup on July 1, 2018.
Russia brought on Aleksandr Erokhin for Daler Kuzyayev in extra time of their last 16 match against Spain.
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What is the significance of a fourth sub?
The impact of late substitutions can be significant, with the potential for a fresh-legged player to trouble a tiring defence and steal the headlines. Indeed, Mario Gotze - who scored the winning extra-time goal for Germany in the World Cup final 4 years ago - was an 88th-minute substitute able to exploit a wilting Argentinian defence.
An extra sub in extra-time will allow managers to not hold back a change in normal time with one eye on the potential for an added 30 minutes. This could result in fewer matches going the additional distance. Meanwhile if extra-time is needed, the introduction of a fourth substitute could provide a game-changing moment and add to the drama.