https://maximummathematics.com/books/foundational-mathematics/set-theory/a-set-of-operations-on-sets/index.htmlWe’ve discussed how to construct new sets by simply taking some of the elements from one set and putting them into a new set. While subsets are vitally important not only in Set Theory but throughout all of mathematics, subsets are formed by only considering one set at a time. We can construct a wide variety of sets by considering two or more sets at a time. In this section, we learn what kinds of sets we can construct by considering more than one set at a time.Hugoen-US