In the Champions League, players receive a one-match ban after collecting four yellow cards. This rule applies from the start of the league phase until the end of the quarter-finals. Once the knockout rounds reach the semi-finals, all yellow cards are wiped out, and players cannot be suspended for accumulated bookings.
The same rules apply to the Europa League and the Conference League, giving players in all three European competitions the same disciplinary framework during the group and early knockout stages.
After the opening two rounds of this season's Champions League, five players had received two yellow cards each. That leaves them two cautions away from a one-match ban as they enter the third round of matches. Players can also be suspended after picking up six, eight or 10 yellow cards. Each time a player reaches an even number of yellow cards above four, he is handed another one-match suspension.
BBC Sport's Ask Me Anything team has outlined these rules to help fans follow which players are at risk of missing matches as the competition progresses. Clubs and supporters are watching closely, since a single booking can decide whether a key player is available for a crucial fixture.
Source: BBC Sport Football · Summarized by HeadlinesBriefing