Products of Open Sets form Local Basis in Product Space

Theorem
Let $T_1 = \left({A_1, \tau_1}\right)$ and $T_2 = \left({A_2, \tau_2}\right)$ be topological spaces.

Let $\left({T, \tau}\right) = T_1 \times T_2$ be the topological product of $T_1$ and $T_2$.

Let $\left({x, y}\right) \in A_1 \times A_2$.

Let $W \in \tau$ be an open set of $T$ such that $\left({x, y}\right) \in W$.

Then:
 * $\exists U_1 \in \tau_1, U_2 \in \tau_2: \left({x, y}\right) \in U_1 \times U_2 \subseteq W$

Proof
Let $W \in \tau$ such that $\left({x, y}\right) \in W$.

From the definition of the product topology, $\tau$ is the topology with basis:
 * $\mathcal B = \left\{{U_1 \times U_2: U_1 \in \tau_1, U_2 \in \tau_2}\right\}$

Thus $W$ is the union of sets of the form $U_1 \times U_2$ where $U_1 \in \tau_1$ and $U_2 \in \tau_2$.

That is:
 * $\exists U_1 \in \tau_1, U_2 \in \tau_2: \left({x, y}\right) \in U_1 \times U_2 \subseteq W$

which is what was to be proved.