[Tex/LaTex] the difference between \em and \emph

Question

What is the difference between \emph{...} and {\em ...}?

When to use each of them?

Answer 1

\emph is like e.g. \textit a command with an argument. \em is the "switch" variant, comparable to \itshape. \em is not an outdated TeX or LaTeX2.09 command but a real LaTeX2e command. Actually \emph is defined through em:

\DeclareTextFontCommand{\emph}{\em}

\em is useful for long texts (\emph e.g. doesn't allow the argument to contain a \par). The commands differ (like the \textit/\itshape) in their handling of the italic correction:

\documentclass{article}
\begin{document}

abc\emph{lll}lll  \textit{lll}llll

abc{\em lll}lll  {\itshape lll}llll

abc{\em lll\/}lll

%\emph{abc\par bc} error

{\em abc\par bc}
\end{document}