Breaststroke is the slowest of the four official styles in competitive swimming, but it is commonly agreed that it is by far the most difficult to do correctly. The history of breaststroke goes back to the Stone Age. The leg action of the breaststroke may have originated by imitating the swimming action of frogs.
Freestyle is an unregulated swimming style used in swimming competitions according to the rules of FINA. Competitors in freestyle swimming can use any of the unregulated strokes such as front crawl, dog paddle, or sidestroke, etc. Most competitive swimmers choose the front crawl during freestyle competitions, as this style provides the greatest speed. As such the term freestyle is often used as a synonym for the front crawl.
The backstroke has the advantage of easy breathing, but the disadvantage of swimmers not being able to see where they are going. It is also the only competition swimming style that starts in the water. Backstroke is an ancient style of swimming. The first Olympic backstroke competition was the 1900 Paris Olympics men's 200 metre.
Butterfly stroke 蝶泳
While other styles like the breaststroke, front crawl, or backstroke can be swum easily even by beginners, the butterfly is a more difficult stroke that requires good technique to be feasible. It is the newest swimming style swum in competition, first swum in 1933. Interstingly, the butterfly kick was developed separately, and is also known as the "dolphin kick".