This weekend, the Premier League and Women's Super League will introduce concussion substitute trials.
The decision comes after the game's rule-making body IFAB last month gave the go-ahead for trials of additional "concussion subs".
Here, Sky Sports explains how the trial period will work.
How have we come to this decision?
The International Football Association Board [IFAB] have been pushing for trial concussion substitutes in the hope of them being permanently introduced to the Laws of the Game.
IFAB believes where there is any doubt about a concussion case, the player or players in question should be protected by being "permanently" removed from the match.
To facilitate this, a team should not suffer a numerical disadvantage by prioritising the player's welfare. The trial was approved at the shareholders' meeting last month with the top flight implementing the plan from matchday 23.
The Premier League will be the first competition in the world to introduce concussion substitutes.
The lunchtime game between Aston Villa and Arsenal on Saturday will be the first fixture under the new regulations. The trial will run in the Premier League until the end of the season but can be extended into the 2021/22 season.
How will the concussion subs work?
During the trial, teams will have two additional permanent concussion substitutions per game [APCs].
If the referee stops a match for a potential concussion injury to a player, the medical staff of that player's team will enter the field of play to make an assessment while the tunnel doctor will review video footage of the incident.
The team doctor will have as much time as is required to make an on-pitch clinical assessment of the player.
In the event of one team having to make a change due to clear symptoms of concussion - or clear video evidence thereof - the opposing team will also be allowed to make an additional substitution at a time of their choosing.
It was agreed the opposing team should be given an additional substitute in this scenario to ensure neither side can claim to have a possible advantage by having more fresh players on the field.
The substituted player will not be allowed to return to the field of play.
But how can we be sure of there not being symptoms?
Should no clear immediate concussion symptoms appear based on the clinical assessment or video evidence, the player will be allowed to continue but will be regularly observed during play by his medical staff.
In the event of clear concussion symptoms later evolving, the player will be replaced through the procedures outlined above.
Even should no clear symptoms develop, the player will undergo a formal clinical assessment at half-time or full-time.
Ref Watch: Green and white card system
Speaking to Sky Sports this week, former Premier League referee Dermott Gallagher outlined how a green and white card system will work.
"To keep it simple, this had to come as player safety is absolutely paramount. We only have to look back a couple of weeks to the David Luiz and Raul Jimenez incident. From Saturday, each team will have two concussion substitutes.
"The fourth official will go to both teams at the start of a game and he will give them three substitute cards in white and two green concussion substitute cards.
"If one team has to make a concussion substitute, they will hand a green card to the referee, and to balance that up, the opposing team will be given a white card so he can use an extra substitute.
"What is means, in essence, is that if for whatever reason one team has to use up two concussion substitutes, the opposing team will get two additional subs. That would mean them having five white cards, as well as two green cards should they need them.
"Theoretically, they could use seven substitutes in one game. We hope and pray that it never has to be used. Nobody wants to see that situation, but we do have a care and a duty of attention to players, and over the last couple of years we have seen some bad head injuries.
"If this does occur on Saturday or going forward, we now have that caveat. It's there to be used and it's a bit like insurance. You have to buy it, but you hope it never has to be used."
What if one team suffer three concussion injuries?
Unfortunately, the maximum number of APCS available as part of the trial is two.
So the team in question would have to either use up a normal substitution or the player must be withdrawn and the team will play the remainder of the game with a player fewer.
Will this only be used in the Premier League and WSL?
No. The new protocol will also come into force in the FA Cup from the fifth round, which will be played from February 9-11, and will be used in the Women's Championship from February 6.
FIFA will also trial concussion substitutes at the Club World Cup in Qatar this month.
What has been said?
The Premier League released a statement following a meeting in December which read: "With player welfare the Premier League's priority, clubs agreed in principle to introduce additional permanent concussion substitutions following approval of the trial by the International Football Association Board.
"The Premier League will look to implement protocols which will allow a maximum of two concussion substitutes to be used per team, with the opposition side able to use the equivalent number.
"The additional concussion substitutions may be made regardless of the number of substitutions a team has made already."
A joint statement from the FA and Premier League last month read: "The trial is a result of the IFAB's consultation with stakeholders and recommendations from their concussion expert group to allow additional substitutions for players with actual or suspected concussion."
Speaking to Sky Sports News, PFA chief executive Gordon Taylor described concussion substitutes as "very important" and said he expects them to be implemented in the English game "sooner rather than later".
"We've made improvements in regulations, particularly with regard to concussion and head injuries," Taylor said. "It's not always been followed and implemented as we would like but it's becoming increasingly important that we should do all we can.
"People know the dangers of taking part but that's not to say we haven't got to show a real duty of care.
"I think the concussion subs have also highlighted the issue that we can't be too careful with head injuries and any consequent problems.
"I would expect that to be implemented much sooner rather than later, albeit they are looking at a pilot programme. But I think there is increasing willingness to adopt that."
Fulham manager Scott Parker has spoken in favour of introducing enhanced concussion protocols in the game, saying they would help to take the heat off players and medical staff.
"I do think concussion subs may take the pressure off the medical [staff] or the actual sportsmen in the heat of the arena of competition," Parker said.
"What comes with that is a real desperation at times to stay on the pitch or [to say] 'no, I'm going to be alright'. To have that ability to take someone off and then have an option to come back on would be helpful.
"I know, I was that player and I wouldn't have wanted to come off the pitch no matter what. So if someone had said to me, whether that's right or wrong, that's the fact, so definitely if something's in place that can only help."
Former Tottenham defender Jan Vertonghen revealed last December he played through concussion symptoms for nine months as he fought to earn a fresh contract.
Vertonghen was injured in Spurs' Champions League semi-final first leg against Ajax in 2019 at the Tottenham Hotspur Stadium after clashing heads with team-mate Toby Alderweireld.
The defender was allowed back into the game after prolonged checks by the Spurs medical team on the sideline, but as he realised his symptoms were too severe to continue, Vertonghen quickly signalled to the bench to be taken off.
"Many people don't know this, but [the head injury against Ajax] affected me for a really long time. I had dizziness and headaches," he told Belgian outlet Sporza. "The other day, there was the story about David Luiz and (Raul) Jimenez, in which Jimenez suffered a fractured skull.
"With me, it was my nose and I continued to play, which I shouldn't have done, according to the doctors. In the end, I think I suffered from that head injury for about eight or nine months. That was the reason why I didn't play well.
"I had a year left on my contract and I thought I had to play because I had to showcase myself to other clubs and to Tottenham, but when I played, I was rubbish. I just couldn't produce a good performance. Not many people knew that.
"That was my own choice and not a criticism of anyone else. After five months, there was a day when I started to feel better. When I see the footage from that time, I know when I didn't feel well by looking at myself."
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