Why a Swiss and not an all-play-all
An all-play-all (round robin) is fair but expensive: for 100 players you would need 99 rounds. Impossible for a weekend tournament. A knockout is fast, but half the field goes home after round one. The Swiss is the golden mean: everyone plays the same small number of rounds, nobody is eliminated, and the final standings still rank the field faithfully.
How the pairings are made
The key rule: you pair players with the same (or the closest possible) score, winners against winners, losers against losers. That way the opponents become more and more evenly matched with every round.
Additional rules the software respects:
- Never the same opponent twice.
- Colours are balanced, so that you play roughly the same number of games with White and Black, and do not get the same colour three times in a row.
- Within a group on the same score, the top half is usually paired against the bottom half.
An example first round with 8 players: they are ordered by rating and cut in half, so #1 plays #5, #2 plays #6, and so on. After the round everyone on 1 point forms one group and everyone on 0 another, and the pairings are made inside the groups.
The bye
If the number of players is odd, one player in each round is left without a pairing and receives a bye, as a rule a full point without playing a game. It goes to the lowest-placed player who has not had one yet. If you know in advance that you will miss a round, you can ask for a half-point bye.
Tie-breaks: when the scores are equal
Several players often finish on the same score. Then additional criteria decide:
- Buchholz: the sum of the scores of all your opponents. A higher Buchholz means you played against stronger competition, which fairly rewards the harder road.
- Sonneborn-Berger: the sum of the scores of the opponents you beat, plus half the scores of those you drew with. It rewards wins over strong players.
The order of the criteria is announced by the organiser in advance, in the tournament regulations.
How many rounds for how many players
A rough calculation: you need roughly as many rounds as it takes to "halve" the field down to a single winner, that is log2 of the number of players.
- up to 16 players -> 4-5 rounds
- 30-60 players -> 6-7 rounds
- 100+ players -> 9 rounds (the standard for large opens)
Too few rounds and you get a crowd sharing first place; enough rounds and the top of the table separates naturally.