THE away goals rule has become the most popular way of deciding a football match that is tied after two legs.
It was introduced by Uefa in 1965 and has remained in tournaments ever since.
And below is everything you need to know about a rule that changed football.
What is the away goals rule?
The away goals rule is the most commonly used way of deciding two-legged football.
If the scores are level after 90 minutes at home and 90 minutes away then, rather than go to penalties, the team that scored most goals in the away game goes through.
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For example, in last year’s Champions League semi-final, Atletico Madrid beat Bayern Munich 1-0 at home and lost 2-1 away, but went through on away goals.
The idea is to reward away teams who choose to play attacking football.
Do away goals really count double?
That expression is supposed to be the law, but it’s really more of a figure of speech.
It simply means that away goals are the first decider for a tied two-legged match.
If anything, it is like an extra half a goal for the team that scored more away from home.
Which competitions use the away goals rule?
The Champions League and the Europa League, as well as all South American competitions, use the away goals rule to decide ties that are level after two legs.
The League Cup uses a modification of the rule in its two-legged semi-finals. Rather than imposing the rule after 180 minutes, the teams play extra time before away goals come into play.
An example would be the 2014 semi-final between Manchester United and Sunderland. United lost 2-1 at the Stadium of Light but were 1-0 up at the full time whistle at Old Trafford.
Under UEFA rules that would have meant they won on away goals.
If United had held on throughout extra-time, away goals would have applied and they would have gone through, but instead both teams scored and Sunderland won a penalty shootout.
The Football League play-offs used to use the same system as the League Cup, but the rule was scrapped in 1999 after an initiative from then Ipswich chairman David Sheepshanks.
Ipswich went out of the 1997 and 1999 play-offs on away goals despite finishing 3rd and 4th respectively.
Sheepshanks successfully argued that it was unfair that the lower placed teams benefit from the rule.
The World Cup does not have away goals because all matches at the tournament are one-off games on neutral territory.
However, it does use away goals for qualifying playoffs.
The FA Cup doesn't use away goals because ties are not played over two legs.
Is the away goals rule popular?
The rule has survived for 51 years by generally being seen as the fairest way to decide a two-leg match.
But it has prominent critics such as Sir Alex Ferguson and Sepp Blatter, who say it was designed for a different age.
They believe the rule was only appropriate for the longer away trips of decades ago, where games would be played on dreadful pitches and playing at home was a key advantage.
The UEFA version of the rule is also believed, by some, to be unfair because the team that plays away second can get 30 minutes more to score an away goal than the home team if the tie goes to extra-time.
Occasionally, there is even the bizarre scenario of two matches at the same stadium being decided on away goals, for example when AC Milan knocked Inter out of the Champions League in 2003.
The first leg at the San Siro, when AC Milan were technically at home, finished 0-0.
And the second leg finished 1-1 at the same venue, which meant Inter qualified.