The away goals rule is not a difficult concept to understand, but its use can vary from competition to competition.
Thankfully, in Uefa's flagship tournaments, the rule is applied in a simple fashion.
What is the away goals rule?
In the Champions League and Europa League competitions, if two teams are level on aggregate after 180 minutes of football, the team who has scored the greater proportion of their goals away from home progresses.
For example, Manchester City were eliminated from this season’s Champions League despite drawing 6-6 with Monaco on aggregate.
Pep Guardiola’s side won the first leg at home 5-2 but suffered a 3-1 defeat away. They had conceded twice at Etihad and scored just once on Monaco’s patch, so they were knocked out on away goals.
If both teams have scored the same number of goals home and away after 180 minutes of football, then a result cannot be determined via the away goals rule. A standard, additional 30-minute period of extra time is played.
Do away goals count in extra time?
Yes. In both the Champions League and Europa League, away goals still count in periods of extra time.
If the visiting team takes the lead in extra time under the away goals rule, the hosts must then score twice in order to progress.
Chelsea were knocked out of the 2014/15 Champions League by Paris Saint-Germain on away goals in extra time.
Eden Hazard’s 96th-minute penalty had handed Jose Mourinho’s men a 3-2 aggregate lead, but David Luiz’s header 18 minutes later sent the visitors through on away goals.
Do away goals 'count double'?
This is a figure of speech which only serves to cause confusion about a rather simple rule.
Away goals in and of themselves do not 'count double'. If a visiting team is losing 6-0 on aggregate but suddenly pulls one back, the score does not become 6-2.