What makes a butterfly stroke legal?
B · Beginner · The stroke · 2 of 6In butterfly both arms must be brought forward over the water at the same time and pulled back under the water at the same time. The legs must move up and down together. They need not be level, but they may not alternate. A breaststroke kick is not allowed. The body stays on the breast.
How it works
- On the breast: "From the beginning of the first arm stroke after the start and after each turn, the Athlete's body must be on the breast."
- No rolling over: it is not permitted to roll onto the back at any time, except at the turn after the touch.
- The arms: "Both arms must be brought forward simultaneously over the water and brought backward simultaneously under the water throughout the race".
- The legs: "All up and down movements of the legs must be simultaneous."
- Not level, not alternating: "The legs or the feet need not be on the same level, but they must not alternate in relation to each other."
- No breaststroke kick: "A Breaststroke kicking movement is not permitted."
- Under water: after the start and each turn the swimmer may take one or more kicks and one arm pull under water, and that pull must bring them to the surface.
Tricky cases
A tired swimmer's arms drag through the water on the way forward. Is that a fault?
Yes, if the arms are not brought forward over the water. The recovery must be over the surface.
One arm enters the water before the other. Is that a fault?
Yes. The arms must be brought forward at the same time.
One foot is a little higher than the other in the kick. Is that a fault?
No. The legs and feet need not be on the same level. They may not alternate.
Can a butterfly swimmer breathe to the side?
Yes. The rule says nothing about how the swimmer breathes, as long as the body stays on the breast.
Why the rule exists
Butterfly was once a way of swimming breaststroke with the arms recovering over the water. When it became its own stroke, the rules had to hold it apart from breaststroke on one side and front crawl on the other. Both arms over the water at once does the first. Legs that never alternate does the second.
Where it differs
| Where | What changes | Source |
|---|---|---|
| World Aquatics rules | As above | SW 8.1 to 8.4 |
| Breaststroke | The hands go forward from the breast and one butterfly kick is allowed after the start and each turn | SW 7.1; SW 7.4 |
| At the turn | After the touch the swimmer may turn in any manner | SW 8.2 |
From the rule book
World Aquatics, Competition Regulations, Part Two: Swimming Rules (in force as from February 2026), Articles 8.1 to 8.4 and 8.6. Checked 14 October 2026. Official document: World Aquatics Competition Regulations (PDF).