[Tex/LaTex] the code for the Halmos tombstone

Question

I have a quote where Halmos writes "The symbol▐ is used throughout …". What is the tex-code for the Halmos tombstone, and what package is needed for it?

Answer 1

Perhaps \RectangleBold or \Rectangle from the bbding package? Or make your own symbol using a \rule (no packages required):

\documentclass{article}
\usepackage{amsthm}
\usepackage{bbding}

\newcommand\MyRectangle{\rule{.36em}{2ex}}

\renewcommand\qedsymbol{\RectangleBold}

\begin{document}

\begin{proof}
A test text.
\end{proof}

\renewcommand\qedsymbol{\Rectangle}

\begin{proof}
A test text.
\end{proof}

\renewcommand\qedsymbol{\MyRectangle}

\begin{proof}
A test text.
\end{proof}

\end{document}