Paragraph numbers at left margin

Question

I would like to number some paragraphs of some sections inside a big document as described in https://stackoverflow.com/questions/543208/how-can-one-number-paragraphs-in-latex, see below:

\newcounter{parnum}
\newcommand{\N}{%
   \noindent\refstepcounter{parnum}%
    \makebox[\parindent][l]{\textbf{[\Alph{parnum}].}}}
% Use a generous paragraph indent so numbers can be fit inside the
% indentation space.
\setlength{\parindent}{2em}

This numbers the paragraphs I wanted, by putting \N before the desired paragraphs.

But I would like the paragraph numbers [A], [B], [C] to be in the left margin, while the paragraphs rest indented normally.

Do you have any suggestions?

Answer 1

One could use the following definition of \N to achieve what you want:

\documentclass{article}

\usepackage[nopar]{lipsum}% Just for this example

\newcounter{parnum}
\newlength{\parnumwidth}
\setlength{\parnumwidth}{3em}
\newcommand{\N}{%
  \noindent\refstepcounter{parnum}%
  \makebox[0pt][r]{\makebox[\parnumwidth][l]{\textbf{[\Alph{parnum}].}}}%
  \hspace*{\parindent}\ignorespaces}
\setlength{\parindent}{2em}

\begin{document}

\lipsum[1]

\N\lipsum[2]

\lipsum[3]

\end{document}

The paragraph number is placed in the margin using a "\llap" which is placed inside a box of width \parnumwidth (set to 3em).

You could also consider using a \marginpar, which allows you to place content in the margin in a more flexible way:

\documentclass{article}

\reversemarginpar% Keep \marginpar in left margin
\newcounter{parnum}
\newcommand{\N}{%
  \leavevmode\refstepcounter{parnum}%
  \marginpar{\textbf{[\Alph{parnum}].}}}%
\setlength{\parindent}{2em}

\begin{document}

\N Lorem ipsum dolor sit amet, consectetur adipiscing elit. Phasellus maximus tortor id suscipit sodales. 
Aenean dui orci, congue in quam id, venenatis dignissim felis. Proin leo massa, convallis eu purus ut, 
bibendum iaculis odio. Sed et lacinia nibh. Duis rhoncus rhoncus lorem, vel sodales est feugiat id. 
Maecenas consectetur dolor vitae sem elementum tincidunt. Nunc in posuere purus. Sed congue ullamcorper 
felis, eget efficitur eros maximus in. Nam placerat justo risus, mollis fermentum velit tempus vestibulum. 
Vivamus placerat neque eu dolor rhoncus blandit. Ut aliquet enim eget nisl luctus euismod. Vivamus 
laoreet, elit eu aliquet bibendum, \N magna lorem efficitur turpis, ac cursus tellus tellus non magna. 
Nullam justo erat, imperdiet vel diam eu, auctor luctus dui. Fusce eleifend sem a pulvinar suscipit. 
Praesent facilisis, velit eget placerat mollis, magna magna venenatis est, at fringilla nulla purus sed dolor.

\end{document}

You may adjust the placement, if needed.