\begin{definition}[Definition:Letter T]
The '''letter $\mathsf T$''' is the topological subspace of the real number plane $\R^2$ under the Euclidean topology defined as:
:$\mathsf T := \paren {\closedint {-1} 1 \times \set 0} \cup \paren {\set 0 \times \closedint 0 1}$
:300px
\end{definition}