How does a Swiss tournament system work?
Players are paired each round against someone on the same score, nobody is eliminated, and everyone plays every round — which lets a large field be ranked meaningfully in far fewer rounds than a round robin would need.
The problem it solves. A round robin, where everyone plays everyone, is the fairest format and requires an impractical number of rounds for a large field. A knockout is fast and eliminates strong players early on one bad result, and tells you nothing about the rest of the field's order. Swiss sits between: it produces a reliable ranking of the whole field in roughly the number of rounds needed to separate the top — commonly around the base-2 logarithm of the field size.
How pairing actually works:
Round one is paired by rating or seeding, typically splitting the field in half and pairing the top of one half against the top of the other.
Subsequent rounds group players by score, then pair within each score group — winners meet winners, losers meet losers.
Two players may not meet twice, which is the constraint that complicates everything and forces pairings across score groups.
Colour or side allocation is balanced, so nobody plays too many games as one colour, and this constrains pairing further.
Floaters — players moved up or down a score group because their group has an odd number — which is unavoidable and is handled by rules about who floats.
Byes where the field is odd, usually to the lowest-scoring player who has not had one.
How ties are broken, which matters enormously since Swiss produces many equal scores. Tiebreaks measure the strength of opposition faced — because two players on the same score have not played the same opponents. Common systems sum the scores of a player's opponents, sometimes discarding the highest and lowest.
Where it is used: chess by origin, and now widely in esports, collectible card games, bridge, Scrabble and debating.
The known weaknesses: accelerated pairings are sometimes used to separate leaders faster; late-round pairings can create situations where a draw suits both players; and pairing software is effectively mandatory, since manual pairing at scale is impractical.