
The group winners and runners-up advance to the round of 16, the first round of the knockout phase. The 32 teams compete in a double round-robin format called the group stage. Then, a team is drawn from each pot until there are eight groups of four.
Pots 2, 3 and 4 contain the remaining teams, seeded based on their 2018 UEFA club coefficients. Pot 1 contains the Champions League titleholders, the Europa League titleholders and the champions of the top six associations based on their UEFA coefficients. If either the Champions League or Europa League titleholders were one of the champions of the top six associations, the champions of the association ranked seventh, and possibly eighth, are also seeded into Pot 1. For the draw, the teams were seeded into four pots based on the following principles: The 32 teams are split into four different "pots," with the restriction that teams from the same association could not be drawn against each other. Each of the top 12 ranked associations in the UEFA coefficients are guaranteed at least one spot, with more spots being allocated the higher a league's coefficient is. The other 22 teams qualified automatically, either as the defending UCL champion, the defending UEL champion or through their respective domestic leagues. F ive teams from each qualifying stream earn a place in the group stage. The two streams are divided between teams qualified by virtue of being a domestic league champion, and those qualified by virtue of being second through fourth in their domestic league. Since the 2009-10 season, the UEFA Champions League offers two qualification "streams" for teams that do not receive direct entry to the tournament. The higher an association's coefficient, the more teams represent the association in the Champions League and the fewer qualification rounds the association's teams must compete in. These scores are generated by the results of clubs representing each association during the previous five UEFA Champions League and UEFA Europa League seasons.
The number of teams each association enters into the UEFA Champions League is based upon the UEFA coefficients of the member associations. The winner and runner-up of each group advanced to the round of 16, where the teams would then continue the usual knockout -stage process (round of 16, quarterfinals, semifinals and final). The 32 teams were split into eight groups of four, with each team playing the other three teams in their respective group in a double round-robin format. In 1992, UEFA (Union of European Football Associations) changed the name of the tournament to the UEFA Champions League and replaced the first round with a group stage. In 1960, the tournament expanded to 32 teams (adding an additional round) and kept the rest of the format the same. Real Madrid defeated Reims 4-3 to win the title in the first tournament. The inaugural tournament, then known as the European Cup (up until 1992), had just 16 teams competing in four knockout rounds (first round, quarterfinals, semifinals and final).
How has the tournament changed since 1955?