Swiss-system tournament

A Swiss-system tournament is a competition format in which contestants are paired using rules designed to ensure that each competitor plays opponents with a similar running score without playing the same opponent more than once. Match pairing for each round is done after the previous round has ended and depends on its results. The winner is the competitor with the highest aggregate points earned in all rounds. With an even number of participants, all competitors play in each round. It contrasts with an elimination tournament, where not all participants play in later rounds, as well as with round-robin tournaments, where each competitor plays every possible opponent.

The Swiss system is used for competitions in which it is undesirable to eliminate any competitors before the end of the tournament, but which have too many entrants to make a full round-robin (all-play-all) feasible. In contrast, all-play-all is suitable if there are a small number of competitors. The Swiss system seeks to provide a clear winner among a large number of competitors within a relatively small number of rounds of competition, while avoiding the situation in single-elimination (knockout) tournaments in which a single bad result can remove a good competitor.

The system was first employed at a chess tournament in Zürich in 1895 by Julius Müller, hence the name "Swiss system", and is now used in many games including bridge, chess, and Go, among others.

The number of rounds is fixed in advance. During all but the first round, competitors are paired based on approximately how they have performed so far. After the last round, players are ranked by their score. If players remain tied, a tie-break score is used, such as the sum of all opponents' scores (Buchholz system).

In the first round, competitors are paired either randomly or according to some pattern that has been found to serve a given game or sport well. If it is desired for top-ranked participants to meet in the last rounds, the pattern must start them in different brackets, just the same as is done in seeding of pre-ranked players for a single elimination tournament. In subsequent rounds, competitors are sorted according to their cumulative scores and are assigned opponents with the same or similar score up to that point. The pairing rules have to be quite complicated, as they have to ensure that no two players ever face each other twice, and to avoid giving a player some advantage as a result of chance.

The detailed pairing rules are different in different variations of the Swiss system. As they are quite complicated, and it is undesirable to have a long delay between rounds to decide the pairings, the tournament organizer often uses a computer program to do the pairing.

In chess, a specific pairing rule, called "Dutch system" by FIDE, is often implied when the term "Swiss" is used. The Monrad system for pairing is commonly used in chess in Denmark and Norway, as well as in other sports worldwide. These two systems are outlined below.

The players are divided into groups based on their scores. Within each group with the same or similar score, players are ranked based on ratings or some other criteria. Subject to the other pairing rules, the top half is then paired with the bottom half. For instance, if there are eight players in a score group, number 1 is paired with number 5, number 2 is paired with number 6 and so on. Modifications are then made to prevent competitors from meeting each other twice, and to balance colors (in chess). For this method to work, the score groups cannot be too small, and thus for smaller overall fields score groups are not a suitable approach.