I would like to show the footnote rule with the same indentation as the footnote. I have tried with something like

\renewcommand\footnoterule{%
\kern-3\p@
\rule{.4\columnwidth}{0.4pt}
\kern2.6\p@}


But it produces inconsistent results. In some cases the rule is indented, in other cases it appears in the same line as the last line of text in the page.

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You have to avoid going in horizontal mode, which \rule does.

\documentclass[a4paper]{article}
\makeatletter

\newlength{\@fnruleindent}
\AtBeginDocument{
\settowidth{\@fnruleindent}{\textsuperscript{1}}
\setlength{\@fnruleindent}{-\@fnruleindent}
}
\def\footnoterule{%
\kern -3\p@
\moveright\@fnruleindent\vbox to.4\p@{\hrule \@width .4\columnwidth}
\kern 2.6\p@
}
\makeatother

\begin{document}
abc\footnote{abc}
\end{document}


This works for footnote numbers up to 9. If you have more footnotes use

\settowidth{\@fnruleindent}{\textsuperscript{11}}


However I don't think it's a good idea.

According to Tschichold, the footnote rule should be the whole line width or absent. Other typographers disagree.

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