As a case in point, consider the example of a four-plane division composed of two sections. Each section might fly welded wing, but the two elements can cooperate as in double attack. For instance, one section could attack an enemy formation while the other section provided defensive "top cover." This combination of fighting wing and double attack doctrines is commonly known today as "fluid four," and it has been widely used by many air services from the time it was first introduced by Werner Moelders during the Spanish Civil War, right up to the present.
...и так далее, в книжке Шоу, глава Division Tactics.