\begin{definition}[Definition:Ordering on Extended Real Numbers]
Let $\overline \R$ denote the extended real numbers.
Extend the natural ordering $\le_\R$ on $\R$ to $\overline \R = \R \cup \set {+\infty, -\infty}$ by imposing:
:$\forall x \in \overline \R: -\infty \le x$
:$\forall x \in \overline \R: x \le +\infty$
That is, considering the relations $\le$ and $\le_\R$ as subsets of $\overline \R \times \overline \R$:
:${\le} := {\le_\R} \cup \set {\tuple {x, +\infty}: x \in \overline \R} \cup \set {\tuple {-\infty, x}: x \in \overline \R}$
where $\tuple {x, +\infty}$ and $\tuple {-\infty, x}$ denote ordered pairs in $\overline \R \times \overline \R$.
The ordering $\le$ is called the '''(usual) ordering on $\overline \R$'''.
\end{definition}