A Swiss-system tournament is a non-eliminating tournament format that features a set number of rounds of competition, but considerably fewer than that of a round-robin tournament. In a Swiss tournament, each competitor (team or individual) does not necessarily play all other entrants. Competitors meet one-on-one in each round and are paired using a set of rules designed to ensure that each competitor plays opponents with a similar running score, but not the same opponent more than once. The winner is the competitor with the highest aggregate points earned in all rounds. All competitors play in each round unless there is an odd number of players.
A Swiss system is used for competitions in which the number of entrants is considered too large for a full round-robin to be feasible, and eliminating any competitors before the end of the tournament is undesirable. Round-robin pairings are suitable for a small number of competitors and rounds, as most or all players will play each other; the underlying assumption is that the player who has played all possible opponents and ends with the highest score must be the winner. Single-elimination (knockout) pairings rapidly reduce the number of competitors, but they may not necessarily result in the best possible competitor winning, as good competitors might have a bad day or eliminate and exhaust each other if they meet in early rounds. Swiss systems intend to provide a clear winner with a limited number of rounds and a potentially unlimited number of opponents. A Swiss system draw should result in a clear winner, without having to play all opponents as in round robin, and without a single bad result terminating participation.
The system was first employed at a chess tournament in Zurich in 1895, hence the name "Swiss system"
The first round is either drawn at random or seeded according to some prior order, such as rating (in chess) or recent performance. All participants then proceed to the next round in which winners are pitted against winners, losers are pitted against losers and so on. In subsequent rounds, each competitor faces an opponent with the same, or almost the same, cumulative score. No player is paired with the same opponent twice. In chess, the pairing rules try to ensure that each player plays an equal number of games with white and black, alternate colors in each round being the most preferable, and a particular effort is made to not assign a player the same color three times consecutively.
During all but the first round, competitors are paired based on approximately how well (or poorly) they have performed so far. 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. Some adjustments may be made to assure that no two players ever oppose each other twice, or to even out advantages a player may have as a result of chance.
The detailed pairing rules are different in different variations of Swiss system. They may be quite complicated, so to make the task easier, quicker and more accurate, the tournament organizer often uses a computer program to do the pairing.