Liverpool face Barcelona tonight at Anfield looking to overturn a 3-0 defeat from the first leg.
Barcelona emerged with the impressive victory at the Nou Camp to put them one foot in the Madrid final next month with Lionel Messi hitting a double.
The Reds are without Mohamed Salah and Roberto Firmino after a disappointing first leg where they failed to find the net and get an away goal.
Divock Origi opened the scoring after seven minutes to give Liverpool hope.
Here's how the away goal rule could come into play if Barca score tonight.
Live match updates from Liverpool vs Barcelona HERE
The away goals rule has been used in European competition since 1965 and it comes into the Champions League for the knockout stages.
Should Barcelona score once, Liverpool will need to score five times to progress, making it 5-4 on aggregate.
Barca would have the away goal advantage, meaning the Reds will be knocked out if the two teams are drawing on the final whistle.
The only way we'd get extra time and possibly penalties is if Liverpool win 3-0.
The away goal rule does count in both periods of extra time. If Barcelona score in extra time, Liverpool score twice in order to progress.
