Competitive swimming is one of the most popular Olympic sports, and also one of the simplest to understand: swim from one end of the pool to the other faster than everyone else. The depth comes from the four different strokes, the strict rules about how each one must be swum, and races that are regularly decided by hundredths of a second.

The four strokes

There are four recognised competitive strokes, plus the individual medley, which combines all four:

• Freestyle — the fastest stroke. Swimmers may use any stroke in a freestyle event, but almost everyone uses the front crawl: alternating arms over the water with a flutter kick. Freestyle events range from the 50-metre sprint to the 1500-metre distance race.
• Backstroke — swum on the back with alternating arms and a flutter kick. Swimmers start in the water, holding the wall, and must finish the race on their back.
• Breaststroke — the oldest of the strokes. Both arms move together in a symmetrical pattern and the kick is a simultaneous frog-style kick. At turns and at the finish, both hands must touch the wall together.
• Butterfly — both arms recover over the water together, powered by an undulating dolphin kick. Like breaststroke, it requires a two-hand touch at turns and the finish.

The individual medley (IM) asks one swimmer to swim all four strokes in a set order: butterfly first, then backstroke, then breaststroke, and finally freestyle. In Olympic competition the IM is swum over 200 and 400 metres.

Pools and distances

The international standard is the long-course pool: 50 metres long, which is the pool used at the Olympics. Short-course pools are 25 metres long and are used for many indoor meets, including the short-course World Championships. Times in the two pool lengths are not directly comparable, because a short-course race has more turns, and turns are where swimmers gain speed from the wall.

The standard individual events are:

• Freestyle: 50, 100, 200, 400, 800 and 1500 metres
• Backstroke, breaststroke and butterfly: 50, 100 and 200 metres
• Individual medley: 200 and 400 metres

Relays add a team element. In a freestyle relay, four swimmers each swim the same stroke. In a medley relay, the four swimmers each swim a different stroke in a fixed order: backstroke first, then breaststroke, then butterfly, and freestyle last. The standard Olympic relays are the 4x100-metre and 4x200-metre freestyle and the 4x100-metre medley.

Open-water swimming is the other side of the sport, raced in seas, lakes or rivers rather than pools, with a marathon swim on the Olympic programme.

Rules that decide races

Each stroke has its own technical rules, and breaking them means disqualification, however fast the swim was. The rules cover the start, the turns, the stroke technique and the finish, and they exist so that no swimmer gains an unfair advantage by swimming a stroke the wrong way.

Starts are from a diving block for freestyle, breaststroke and butterfly, and from the water for backstroke. After the start and after every turn, swimmers may swim underwater for a limited distance before they must surface and swim the stroke. False starts are punished, and in most major competitions a single false start means disqualification.

Finishes are judged by the touch on the wall, and electronic timing pads record results to a hundredth of a second. That precision matters: Olympic finals are regularly decided by margins the eye cannot see.

Formats and major competitions

The sport is governed internationally by World Aquatics, formerly known as FINA. Its two biggest events are the Olympic Games and the World Aquatics Championships, both held in 50-metre pools. The World Cup circuit is raced mainly in short-course pools, and national championships and invitation meets fill the calendar between the major events.

The Olympic swimming programme is one of the largest at the Games, with men's and women's events across every stroke and distance. The 50-metre freestyle is the purest sprint in the sport, over in around twenty seconds, while the 1500-metre freestyle is a fifteen-minute test of pacing and endurance.

A brief history

People have raced in water for thousands of years, but modern competitive swimming took shape in nineteenth-century England, with the first organised races and clubs. The international federation, now World Aquatics, was founded in 1908, and swimming has been part of every modern Olympic Games. Women's swimming joined the Olympic programme in 1912. The strokes themselves have evolved steadily: butterfly split away from breaststroke as a separate stroke, backstroke technique changed with the flip turn, and training and underwater technique have made the sport faster in every generation.

How to follow it

Watch the turns and the underwater phase after them, because that is where races are often won. In a 100-metre race in a 50-metre pool there is only one turn, so the start matters enormously; in a 200-metre race the swimmer who holds their technique together in the final length usually wins. And when two swimmers touch together, wait for the official time — the result is often closer than it looked.