From prelims to final
Each event starts with a preliminary round of heats in the evening. In events 200m and shorter, the 16 fastest swimmers from prelims advance to the semifinals, which are held during the morning session on the following day. The eight fastest swimmers from the two semifinal heats advance to the final, held the following morning. In the event of a tie for 16th in prelims or for eighth in the semifinals, tied competitors swim an additional head-to-head race to determine who advances. In events 400m and longer (including relays), the eight fastest swimmers (or teams) from prelims advance directly to the final. Finals are held the morning after prelims, but for the women's 800m freestyle and the men's 1500m freestyle, the finals are conducted the two days later. In the event that swimmers (or teams) tie for eighth place in prelims, there is a swim-off to determine which swimmer or team advances to the final.
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:
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.
Each country is allowed to 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.
The 10k open water race has only a final, which will be held in the morning at the Shunyi Olympic Rowing-Canoeing Park in Beijing. The 25 swimmers begin at the start of the course and swim four laps around the rowing basin.