In short
The answer first
A single-leg knockout is decided from one match, using extra time or penalties if the competition requires a winner and the score remains level. A two-leg tie gives each club a home match and combines the goals from both legs into an aggregate score before any further deciding procedure is used.
Key takeaways
Single-leg ties concentrate the entire result into one fixture.
Two-leg ties combine scores from a first and second leg into an aggregate total.
The score in one leg and the aggregate score are different pieces of information.
UEFA no longer uses the old away-goals rule to separate level aggregate ties in its men's club competitions.
Competition regulations determine whether extra time, penalties or another method follows a level tie.
What a single-leg tie changes
In a single match, there is no future leg in which to repair a poor result. Teams make decisions knowing that the winner advances that day, which can increase the value of game-state management, substitutions and extra-time planning.
Venue selection can also matter. Some competitions use a predetermined neutral venue for a final, while earlier single-leg rounds may assign home advantage under a draw or seeding rule.
How a two-leg tie works
Each team normally hosts one match. The goals from both fixtures are added together, producing the aggregate score. A team can lose the second leg and still advance if the advantage built in the first leg remains large enough.
That creates two simultaneous game states in the return match: the score on the night and the score across the tie. Broadcasts often display both because tactical decisions respond to the aggregate position.
Level after two legs
The competition regulations specify what happens if aggregate goals are equal after the second leg. In current UEFA men's club knockout ties, the old away-goals rule is not used; the deciding route can move to extra time and, if still level, penalties under the applicable rules.
This is why an away goal should not be described as automatically 'worth double'. It counts as one goal in the aggregate total unless a particular competition explicitly says otherwise.
Why tactics can differ across the formats
A two-leg tie allows teams to manage risk across a longer competitive window. A narrow first-leg defeat can leave a realistic route back at home, while a large first-leg lead changes how aggressively the leading team needs to attack the second match.
Single-leg football removes that second opportunity. The format does not dictate whether teams play attacking or defensive football, but it changes the consequences of each decision and the amount of time available to correct it.
Source check
Sources and further reading
KickVerse uses primary governing-body material where a rule or competition format has an official source. Analysis terms can vary between data providers, so provider-specific definitions are identified rather than presented as universal laws.
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