Knockout football needs a winner, and when two legs cannot separate two Champions League teams, the competition has a clear sequence for deciding the tie. Extra time comes first, then a penalty shootout. The rules are simple, but the details — substitutions, yellow cards and the order of kicks — are worth knowing before a big European night.

Extra time is played only in knockout ties, and only when the aggregate score is level after the second leg. It consists of two periods of 15 minutes, played in full, with a short break between them but no golden goal: a goal in extra time does not end the tie early, and the team that scores first must still defend its lead for the remainder of the period.

Extra time is not played in the league phase, where a draw is simply a draw. The key point is that extra time is a tiebreaker for matches that must produce a winner, not a routine extension of every game. A final level after 90 minutes follows the same sequence: extra time first, then penalties if the score is still level.

Teams are allowed an additional substitution in extra time, beyond the five normally permitted in the match, which gives coaches a chance to introduce fresh legs or a specialist penalty taker. This extra change is one reason why coaches plan their substitutions carefully during the second leg, saving an option for the additional period if the tie remains level.

Yellow cards carry a small but important quirk. A player who receives a second yellow card in extra time is still sent off, but yellow cards from the match itself are not automatically wiped before extra time begins. Discipline therefore matters deep into a tie: a player already booked who commits a reckless foul in extra time risks leaving his team short-handed for the decisive minutes or the shootout.

If extra time cannot separate the teams, a shootout decides the tie. Each team nominates five takers, who alternate kicks, and the team with more goals after five rounds wins. If one team gains an unassailable lead before all five rounds are complete, the shootout ends early. If the score is level after five rounds, the shootout moves to sudden death: one kick each, round by round, until one team scores and the other misses.

The order of kicks is decided by a coin toss, and the team that wins the toss chooses whether to kick first or second. Going first is often seen as a small advantage, because it puts scoreboard pressure on the opponent, but the effect is modest, and shootouts are regularly won by the team kicking second. Goalkeepers may be substituted before the shootout for a specialist, if the team still has a substitution available.

A shootout compresses a whole tie into a handful of kicks, and that compression is why it feels so intense. A player who was quiet for 120 minutes can become the hero with one kick, and a player who played superbly can become the villain with one miss. The drama is not an accident: it is the product of a system designed to produce a winner when two teams cannot be separated by open play.

For supporters, the best preparation is knowing the sequence in advance. If a tie is level late in the second leg, expect extra time rather than a sudden finish. If extra time is level, expect five rounds of penalties and possibly sudden death. Knowing what comes next does not make it less tense, but it does stop the confusion that used to surround away goals and aggregate arithmetic.