Video Assistant Referee (VAR) causes controversy every week in the Premier League, but how are decisions made and are they correct?
After each weekend, we take a look the most high-profile incidents and examine the process both in terms of VAR protocol and the Laws of the Game.
Possible disallowed goal: Handball by Trossard
What happened: Brighton & Hove Albion were already 3-0 up when Manchester United's Diogo Dalot tried to clear the ball off the line, but could only kick it into Leandro Trossard, and it rebounded into the goal.
VAR decision: No handball, goal stands.
VAR Review: There is no doubt that the clearance hit Trossard on the chest. The question was whether the ball then brushed his arm before it went into the goal.
The VAR, Chris Kavanagh, watched replays of the goal from several angles, but there was no definitive proof the ball hit Trossard's arm. If it had, the goal must be disallowed as you cannot score a goal with your arm/hand, even if the contact is accidental.
In these situations, the VAR is looking for a replay which shows beyond doubt that the ball hit the arm and the on-field decision was incorrect. There was no angle which showed any kind of evidence of the handball, so the VAR was correct to allow the goal.
However, Byram knocked the ball on with his arm before scoring. Unlike with Trossard, this was clear from three different camera angles, so the VAR, Graham Scott, had a simple task to tell the referee to disallow the goal.
VAR overturn: Red card for Ayling
What happened: Arsenal were already 2-0 up when Luke Ayling attempted a tackle on Gabriel Martinelli by the corner flag. The Leeds United player went in with both feet off the ground, but referee Chris Kavanagh only showed a yellow card.
VAR decision: The VAR, John Brooks, advised the referee that the yellow card should be upgraded to a red.
VAR review: The only real surprise is that it took so long for the VAR to advise the red card, and even then the referee needed an extended look at the monitor before changing his decision to a red card.
Ayling was off the ground with both feet, leading into the challenge with his studs showing. It was a worse tackle than that of Granit Xhaka against Manchester City at the start the season, though that red card was shown by the referee and not following a VAR review.
The only thing that could have saved Ayling was that the challenge was not high, which is why Kavanagh initially only cautioned him. But the nature of the challenge, with both feet off the floor, was a clear case of serious foul play which endangers the safety of an opponent.
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Possible red card: Fabinho on Son
VAR decision: No red card, a caution was deemed sufficient.
VAR review: The incident was quickly looked at by the VAR, Darren England, but he judged that a yellow card was a justifiable decision by the referee.
This is the key difference between the Ayling and Fabinho incidents. One of the key considerations of the VAR is whether the card the referee has shown is an incorrect decision within the Laws of the Game. In the Fabinho instance, a caution can certainly be argued for; with Ayling, the nature of the tackle made it difficult to argue against a red card.
Perhaps the bigger issue was that referee Michael Oliver refereed the game with a degree of leniency. It meant that Fabinho escaped a number of fouls which on another day could have received a caution -- so by the time he made this foul on Son it could have been a second yellow card and therefore a dismissal.
However, this won't change how a VAR assesses a challenge when a player isn't yet on a card.
There is no doubt that Fabinho deserved a booking, but there was no throwing action with the arm to create force or an element of brutality. Also, there was no clenched fist from the 28-year-old, which referees use to determine if there was violent intent.
VAR overturn: Loftus-Cheek goal disallowed for offside
What happened: The score was goalless when Ruben Loftus-Cheek thought he had scored from close range.
VAR decision: After a very lengthy VAR review, Jarred Gillett correctly disallowed the goal for offside.
VAR review: The problem wasn't the final decision but the time taken to reach it, and the process Gillett went through. A long time was spent checking the first possible offside against Antonio Rudiger following the near-post flick on.
Then, the VAR had to ascertain whether Romelu Lukaku or Leander Dendoncker touched the ball before it ran through to Loftus-Cheek to score. If it were played by Lukaku, it was a clear offside. If Dendoncker, then the Chelsea scorer could not be offside.
The review would have been much quicker had Gillett concentrated first on the offside against Loftus-Cheek, as there was no doubt about his offside position. All that needed confirming was that the ball was touched by a teammate.
VAR overturn: Penalty to Chelsea
What happened: The score was still goalless in the 52nd minute when Romelu Lukaku went down under a challenge from Romain Saiss. Referee Peter Bankes gave a goal-kick when the ball went out of play shortly afterwards.
VAR decision: Gillett reviewed the challenge and advised the referee it should be a penalty.
VAR review: The foul from Saiss wasn't clear initially, but the replay showed that the defender did catch Lukaku high on the leg. It was the correct decision to award the penalty, which was scored by Lukaku himself.
Information provided by the Premier League and PGMOL was used in this story.