\begin{definition}[Definition:Language of Propositional Logic/Alphabet/Letter]
Part of specifying the language of propositional logic $\LL_0$ is to specify its letters.
The letters of $\LL_0$, called '''propositional symbols''', can be any infinite collection $\PP_0$ of arbitrary symbols.
It is usual to specify them as a limited subset of the English alphabet with appropriate subscripts.
A typical set of '''propositional symbols''' would be, for example:
:$\PP_0 = \set {p_1, p_2, p_3, \ldots, p_n, \ldots}$
\end{definition}