From Heats to Final
Each swimming event is broken up into rounds with the purpose of trimming the field of competitors (which can sometimes number upwards of 90+ for individual events) down to a final race of eight. Individual events at distances of 200m and shorter require three rounds: preliminary heats, semifinals, and a final. Each athlete participates in one of several preliminary heats in hopes of recording a top-16 time, thereby advancing to the semifinals.
From there, the eight fastest swimmers across both semifinal heats advance to the final. Individual events at distances of 400m or longer, along with all relay events, do not include a semifinal round. In these events, only the top eight swimmers or teams across the preliminary heats advance to the final.
In the event of a tie for the final lane in a semifinal or final round, tied competitors swim an additional head-to-head race to determine who advances. This is also known as a "swim off".
Olympic swimming competition days generally contain two sessions: one for only preliminary heats, and a second for semifinals and finals. The exceptions are the first day of competition, which lacks a semifinal/final session, and the last day, which does not contain a heats session.
Lane Assignments
Swimmers and relay teams are seeded according to times from the previous round so that the fastest swimmers/teams are in the central lanes of the pool, and the slowest are in the outer ones.
Example:
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Lane 1: Seventh-fastest
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Lane 2: Fifth-fastest
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Lane 3: Third-fastest
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Lane 4: Fastest swimmer or relay team
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Lane 5: Second-fastest
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Lane 6: Fourth-fastest
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Lane 7: Sixth-fastest
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Lane 8: Eighth-fastest
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Being in the middle of the pool is advantageous because waves reverberating off the side walls can slow a swimmer. Also, swimmers in the middle lanes are better able to monitor other competitors.
Relays
Each country can enter up to eight swimmers in each relay. In many cases, a country will have alternates swim in the prelims, allowing the fastest swimmers to be rested for the final. Athletes who swim in only the heats are awarded a medal if their country earns one in the final, meaning up to eight swimmers can win a medal for a relay.
Open Water
The 10k open water race only has a final, which will be held at Odaiba Marine Park in the Tokyo Games. The event was introduced at the 2008 Beijing Olympics and is sometimes referred to as "marathon swimming" or "the 10k." Twenty-five competitors will swim in the event. There are no lanes, and physical contact between swimmers is expected, especially during the mass-start beginning to the race. Like the running marathon, the open water race follows a prescribed course, often including multiple laps. Similarly, swimmers can stop at a feeding station periodically during the race to hydrate and refuel. There are two 10k races: a men’s race and a women’s race.