Numbering theorems

Question

I am looking for a way for counting theorems and propositions by using letters instead of numbers. Looking for on this site, I have found this code:

\newtheorem{thma}{Theorem}

\newtheorem{propa}{Proposition}

\renewcommand{\thethma}{\Alph{thma}}

\renewcommand{\thepropa}{\Alph{propa}}

However, theorems and propositions are counted separately. Is there a way for a counting like below?

Theorem A

Proposition B

Theorem C

Answer 1

This should be a part of any instruduction that mentions how \newtheorem works. In this case the thma and propa envs define their own separate counters as well. But using the proper syntax you can tell propa to use the same counter as thma

\newtheorem{thma}{Theorem}
\newtheorem{propa}[thma]{Proposition}
\renewcommand{\thethma}{\Alph{thma}}

as both are not using the same counter, there is no \thepropa anymore

Answer 2

This will work:

\documentclass{article}
\usepackage{amsmath}
\newtheorem{thma}{Theorem}
\newtheorem{propa}[thma]{Proposition}
\renewcommand{\thethma}{\Alph{thma}}
\renewcommand{\thepropa}{\Alph{propa}}
\begin{document}
\begin{thma}
  AAA
\end{thma}
\begin{propa}
  BBB
\end{propa}
\end{document}