What are the butterfly rules?

Under World Aquatics rules (Part Two, Article 8):

• From the first arm stroke after the start and each turn, the body must be on the breast. Rolling onto the back isn't allowed, except at the turn after the touch, as long as the swimmer is on the breast by the first arm stroke off the wall.
• Both arms must come forward together over the water and go back together under the water.
• All up-and-down leg movements must be simultaneous. The legs or feet needn't be level but mustn't alternate. No breaststroke kick.
• After the start and each turn, the swimmer may take one or more kicks and one arm pull underwater, which must bring them to the surface, and the head must break the surface by 15 metres.
• Touch at every turn and the finish with both hands separated and simultaneously, at, above or below the surface.