https://maximummathematics.com/books/foundational-mathematics/set-theory/empty-set/index.htmlJust because we can put just about anything in a set doesn’t mean we need to have something in a set. There’s a special and unique kind of set that has no elements in it at all! EMPTY SET, NULL SET The empty set is the unique set containing no elements at all. The empty set is sometimes referred to as the null set, and is often symbolized by $\emptyset$.Hugoen-US