nested enumeration, skipping items

Question

How can I do a nested enumeration with item being skipped, like this:

1) Level 1
 A) sublevel A
  i) sublevel i
   a) sublevel a

2) Level 1 [here are the skipped \item]
   a) sublevel a
   b) sublevel a

3) Level 1
 A) sublevel A
   a) sublevel a
   b) sublevel a
 B) sublevel A
  i) sublevel i
   a) sublevel a
 ii) sublevel i

I tried something like this, unsuccessfully:

\begin{enumerate}
  \item
  \begin{enumerate}
    \begin{enumerate}
     ...
    \end{enumerate} 
  \end{enumerate}
  \item
\end{enumerate}

Answer 1

If you want to skip the use of an itemizing something in a nested list while still maintaining the indentation, you can use \item[] to override the labelling and therefore obtain a label-less \item. The following produces exactly the nesting and labelling you're after:

\documentclass{article}
\usepackage[shortlabels]{enumitem}

\begin{document}
\begin{enumerate}[1)]
  \item Level 1
  \begin{enumerate}[A)]
    \item sublevel A
    \begin{enumerate}[i)]
      \item sublevel i
       \begin{enumerate}[a)]
         \item sublevel a
       \end{enumerate}
    \end{enumerate}
  \end{enumerate}

  \item Level 1 [here are \textbf{two} skipped sublevel \verb|\item|s]
    \begin{enumerate}[A)]
      \item[]% Empty item (nesting kept)
      \begin{enumerate}[i)]
        \item[]% Empty item (nesting kept)
        \begin{enumerate}[a)]
          \item sublevel a
          \item sublevel a
        \end{enumerate}
      \end{enumerate}
    \end{enumerate}

  \item Level 1
  \begin{enumerate}[A)]
    \item sublevel A [here is \textbf{one} skipped sublevel \verb|\item|]
    \begin{enumerate}[i)]
      \item[]% Empty item (nesting kept)
      \begin{enumerate}[a)]
        \item sublevel a
        \item sublevel a
      \end{enumerate}
    \end{enumerate}
    \item sublevel A
    \begin{enumerate}[i)]
      \item sublevel i
      \begin{enumerate}[a)]
        \item sublevel a
      \end{enumerate}
      \item sublevel i
    \end{enumerate}
  \end{enumerate}
\end{enumerate}
\end{document}​

The biggest thing to maintain the structure is to keep your code using a uniform indentation. That way you can easily see at which level certain items are. Of course, (La)TeX has no concern with this as long as your groups are well-ordered. However, debugging code could be messy if the indentation is lacking.

Answer 2

Is this what you mean?

\documentclass{article}
\usepackage[shortlabels]{enumitem}

\begin{document}
\begin{enumerate}[1)]
\item Level 1
\begin{enumerate}[A)]
    \item sublevel A
    \begin{enumerate}[i)]
      \item sublevel i
        \begin{enumerate}[a)]
           \item sublevel a
        \end{enumerate}
    \end{enumerate}
\end{enumerate}
\item Level 1 [here are the skipped item]
    \begin{enumerate}[a)]
       \item sublevel a
       \item sublevel a
    \end{enumerate}

\item Level 1
\begin{enumerate}[A)]
 \item sublevel A
    \begin{enumerate}[a)]
       \item sublevel a
       \item sublevel a
    \end{enumerate}
 \item sublevel A
    \begin{enumerate}[i)]
  \item sublevel i
        \begin{enumerate}[a)]
           \item sublevel a
        \end{enumerate}
 \item sublevel i
    \end{enumerate}
\end{enumerate}
\end{enumerate}
\end{document}

If you meant that you want each of the a) items indented to the same horizontal space, then you can use empty itemize lists which have a \phantom label so nothing gets printed. The parameter of the \phantom would need to be the largest value.

\documentclass{article}
\usepackage[shortlabels]{enumitem}

\begin{document}
\begin{enumerate}[1)]
\item Level 1
\begin{enumerate}[A)]
    \item sublevel A
    \begin{enumerate}[i)]
      \item sublevel i
        \begin{enumerate}[a)]
           \item sublevel a
        \end{enumerate}
    \end{enumerate}
\end{enumerate}
\item Level 1 [here are the skipped item]
    \begin{itemize}[\phantom{A)}]\item% This provides a phantom "A)" list
    \begin{itemize}[\phantom{i)}]\item% This provides a phantom "i)" list
    \begin{enumerate}[a)]
       \item sublevel a
       \item sublevel a
    \end{enumerate}
    \end{itemize}
    \end{itemize}
\item Level 1
\begin{enumerate}[A)]
 \item sublevel A
    \begin{itemize}[\phantom{i)}]\item
    \begin{enumerate}[a)]
       \item sublevel a
       \item sublevel a
    \end{enumerate}
    \end{itemize}
 \item sublevel A
    \begin{enumerate}[i)]
  \item sublevel i
        \begin{enumerate}[a)]
           \item sublevel a
        \end{enumerate}
 \item sublevel i
    \end{enumerate}
\end{enumerate}
\end{enumerate}
\end{document}

Answer 3

The easylist package also allows users to skip levels:

\begin{easylist}
§ First proposition.
§§§ A sub-comment to the first proposition.
\end{easylist}

See the Usage chapter in the documentation for details.