What is the code for the symbol "_|" in the picture below?

Thanks a lot.
What is the code for the symbol "_|" in the picture below?

Thanks a lot.
I think it’s \lrcorner from amssymb
\documentclass[fleqn]{article}
\usepackage{amssymb}
\begin{document}
\[\oint v_\lrcorner \dots \]
\verb|\lrcorner| $\lrcorner$
\verb|\llcorner| $\llcorner$
\verb|\urcorner| $\urcorner$
\verb|\ulcorner| $\ulcorner$
\end{document}

I found it in the LaTeX symbol list, cf. How to look up a symbol or identify a math symbol or character?