Consider the following definition of directed graph
Definition of Directed Graph Using an Ordered Pair of Sets
We define a directed graph to be an ordered pair $(V, E)$ such that $V$ is a finite set of integers and $E$ is a subset of $V \times V$. Note that $V \times V$ is the set $\begin{Bmatrix}(v_{1}, v_{2}): v_{1}, v_{2} \in V \end{Bmatrix}$.
There are a lot of variable names which are only one letter long, such as $V$ and $E$. That gets to be overwhelming when people define tuples of objects such the tuple contains more than five items.
What the the names of some published researchers who write definitions like this:
Definition of Directed Graph Using Classes and Class Attributes
We define a directed graph to be a class $D = (verts, edges)$ such that $D.verts$ is a finite set of integers and $D.edges$ is a subset of $D.verts \times D.verts$.
Note that, $D.verts \times D.verts$ is the set $\begin{Bmatrix}(v_{1}, v_{2}): v_{1}, v_{2} \in V \end{Bmatrix}$