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The Swiss Pairing System
Prior to the 1900s, most chess tournaments were played as a “Round Robin”, where everyone in the tournament played every other person. Although this is probably the best way to determine the overall order, it makes for a very long tournament. For a 20 player event, it would take 19 rounds to play all the games!
In 1895, a pairing system invented by J. Muller was first used in a chess tournament in Zurich, Switzerland (which is why it is called the “Swiss” system.) The main idea of the Swiss system is for players to be matched against other players who have the same score (the same number of wins and losses.) Players alternate playing black and white pieces if possible, and do not play the same person twice.
The advantage of the Swiss system is that it is usually possible to determine a clear winner within a few rounds, even for relatively large groups, and noone is “eliminated” as with bracketstyle pairing systems. The disadvantage is that it often results in many players being tied for the same position in the tournament. . Over time, many tie breaking systems have emerged – see “Chess Tournament Tie Breaking” for an explanation of tie breaking.
To help explain the Swiss pairing process, this document contains examples of tournament play with fictitious names and results, starting simple and moving toward more complex situations.
Unrated Tournament Pairing
At an unrated tournament, first round pairing is by random draw. For example, with 16 players they would be matched into 8 random pairs for the first round. For now, assume all games have a winner, and there are no draws.
After the first round, there will be a group or “pack” of 8 players with a score of 1 (win), and a group of 8 players with a score of 0 (loss). For the 2^{nd} round, players in each scoring group will be paired against each other – 1’s versus 1’s and 0’s versus 0’s.
After round 2, there will be three scoring groups:
Again, for round 3, players are paired with players in their scoring group. After the third round, the typical scoring groups will be:
For the fourth (and in this case final) round, the process repeats, and players are matched with others in their scoring group. Note that there are only 2 players who have won all of their games so far – they will be matched against each other for the “championship” game. After the final round, we’ll have something that looks like this:
So, as mentioned before, the highlights of the Swiss system are that it produces a clear winner in just a few rounds, noone is eliminated and almost everyone wins at least one game, but there are many ties to deal with!
Rated Tournament Pairing
Many chess tournaments account for the strength of players by using a rating system (usually the USCF rating) in the Swiss pairing process. The basic ideas are the same – players with like scores play against each other – but instead of random assignment the pairs are clearly defined. Each player is “seeded” according to their rating, which determines who they play for the first (and often subsequent) rounds.
For this example, let’s assume we have 16 players with the following ratings. They will be seeded 1 through 16, as shown:
For the first round, the top half of the seeded players plays the bottom half in corresponding order. The #1 seeded player plays #9, #2 plays #10 and so forth, usually with alternating colors:
Ann vs. Ivan Judy vs. Bill Chip vs. Kirk (etc.) Pete vs. Hal
As before, after the first round there will be two scoring groups of winners and losers. Then the process repeats within each scoring group, with the top half of a group (by seed) playing the bottom half of that group. The same rules apply to rounds three and four.
If everyone plays according to their rating, the championship game in the fourth round will be between the #1 and #2 seeded players, and after the fourth round is complete, the finishing order will match the original seed order. But, it almost never happens that way in real life – players have good days and bad days, make brilliant moves and spectacular blunders  chess is funny that way!
Then It Gets Messy – Ties, Byes, Mixed Ratings and other Exceptions
The previous examples are simplified to make it easy to understand the basics. In reality, it can get complicated pretty quickly.
One issue that has to be dealt with is the fact that a game of chess doesn’t always have a winner – there can be a draw. The same rules of Swiss pairing apply – players with the same score play against each other. But with draws (which score ˝ point or 0.5), suddenly you can have many more scoring groups. After 3 rounds, there are 7 possible scores (3.0, 2.5, 2.0, 1.5, 1.0, 0.5 and 0) and therefore 7 possible scoring groups! If there are an odd number of players in any particular group, it’s not possible to pair exactly, and players have to be shifted up or down to neighboring groups to make it all work. (The general rule of thumb is if there are an odd number of players in a group, the highest rated player in the next group down moves up to even it out.)
A second common occurrence is that the number of players that show up for tournament isn’t always even. When there is an odd number, there will be one player in each round who doesn’t get matched against another player, and will receive a “bye”. The rules for byes are set by the Tournament Director – the odd player can be given 1 point, half point, or no points for the bye. (In most scholastic tournaments, one point byes are given.) The pairing rules change based on the type of bye given, and are a bit beyond the scope of this example, but in general the bye goes to the highest rated player in the lowest score group, with at most one bye per player in a tournament.
With rated tournaments, often not every player that shows up to play has an established rating. In those cases, unrated players can either be seeded in random order below the last rated player, or given estimated ratings based on their age, experience, or performance at prior tournaments.
Many other variables can make pairing “messy.” For example, two players with the same score that should be matched against each other may have already played each other earlier in the tournament, and will have to be moved around. Sometimes, exceptions have to be made for color choice if one player has already played a certain color too many times. The list goes on and on.
The Tournament Director may decide to include optional pairing restrictions. The most common pairing restriction in scholastic tournaments is by school or club. In that case, players who go to the same school or belong to the same chess club will avoid being paired against each other, if possible.
Advanced Topic  Accelerated Pairing
The Swiss system can produce a clear winner more quickly than Round Robin, but there are limits. The rule of thumb is that it can handle 2^{n} players, where n is the number of rounds. Therefore, 8 players needs 3 rounds, 16 players needs 4 rounds, 32 players needs 5 rounds, and so forth. (These numbers are approximations  due to draws and other variables, sometimes it works with more players than expected.)
If there are more players in a section than the number of rounds can typically handle, "accelerated" pairings are an option for the Tournament Director.
Players are seeded as with a regular rated tournament, but in the first round, the players from the top quarter of the seeding list play the players in the 2nd quarter of the list. The 3rd quarter plays the bottom quarter. Then in the second round, the winners in the top half of the seeding play each other, the losers in the top half play the winners from the bottom half of the seeding, and the losers from the bottom half play each other. (The theory is that the higher rated losers from the top half should beat the lower rated winners from the bottom half, which would cut down the number of perfect scores faster). After the 2nd round, all the players are lumped together within their score groups as in the traditional Swiss system, and the tournament continues with regular Swiss pairing. The only difference is there should be half as many players with a perfect 20 score after the second round as there would have been with a straight Swiss system tournament, so up to 64 players could be accommodated in a 5 round tournament.
Conclusion
Hopefully you found this explanation helpful for answering the oftenconfusing question of why players are matched against each other during chess tournaments. Keep in mind, of course, that the most important aspects of scholastic chess tournaments are to learn, make friends, be a good sport, and have fun.
