Video Assistant Referees (VAR) are being used in the Champions League and Europa League competitions for the first time this season, with the technology already making an impact.
Ajax had a goal controversially ruled out against Real Madrid by VAR’s first telling Champions League contribution, with the Dutch side losing the first leg of their last-16 tie 2-1 on February 13.
As the colossal De Ligt powered a header at goal, Courtois could only parry and Nicolas Tagliafco reacted first to nod home.
Almost three minutes passed, during which the home fans and players celebrated, before Damir Skomina, having consulted his touchline monitor, ruled the goal out.
His decision was that Tadic, marginally in an offside position, had interfered with Courtois’ (probably futile) attempt to block the second header.
So, with VAR failing to remove the controversy surrounding goals or non-goals, here's a reminder of how the system works...
How VAR works
VAR operates on a three-step process of incident, review/advice, decision, and across the four jurisdictions of football deemed 'game changing' by Fifa.
The IFAB says VAR is only to be used "to correct clear errors and for missed serious incidents" in those "match-changing" situations.
VARs and other match officials are able to recommend reviews, but the only person who can initiate one is the referee, who will then have the final say on whether their original decision should stand or be changed.
They have the option to review footage themselves on a pitch-side monitor before making a final decision.
Goals
VARs are used here to help the referee determine whether an infringement was made that should result in the goal not being awarded.
Penalties
Here, VAR will look to ensure that the correct decision was made regarding the award or non-award of a penalty.
Red cards
Much like with penalty decisions, VAR will look to ensure that the correct decision was made.
Mistaken identity
VAR will clarify which players should be cautioned or sent off, or correct the referee in the event of mistaken identity.
Where else will VAR be used?
The technology will be in place at this summer's Nations League Finals - which will feature England as well as Portugal, Netherlands and Switzerland.
Having been trialled at the World Cup in Russia, as well as certain FA Cup and League Cup fixtures in English football, the technology will also be used in the Premier League from next season after clubs agreed the introduction in principle at a meeting in November.
