Definition of a network in Graph Theory (Complex Networks)

Question

I'm trying to understand a more precise definition of a graph by Bollobas:

"A graph $G$ is an ordered pair of disjoint sets $(V, E)$ such that $E$ (the edges) is a subset of the set $V_2$ of unordered pairs of $V$ (the vertices)."

My question is, if $V$ and $E$ are disjoint, how can $E$ be a subset of a set of unordered pairs of $V$.

Am I wrong with understanding some of the properties of structures used here?

Answer 1

There is actually no contradiction. Elements of $V_2$ are "unordered pairs $V$" (here that means "two-element subsets of $V$"), which generally do not belong to $V$ as elements. So $V_2$ and $V$ do not necessarily intersect, and that means, that it is possible for $E$ to be both disjoint with $V$ and a subset of $V_2$.