A frequently seen mistake is to use \begin{center} … \end{center} inside a figure or table environment. This center environment can cause additional vertical space. If you want to avoid that just use \centering instead like in this example:

\begin{figure}[ht]
\centering
\includegraphics{filename}%
\caption{text}%
\end{figure}

The additional space of the center environment is caused by a trivlist environment. Its defined by latex.ltx:

\def\center{\trivlist \centering\item\relax}
\def\endcenter{\endtrivlist}

As you can see \center calls \centering too. By directly using \centering you could omit that trivlist.

Inside normal text \begin{center} … \end{center} is useful of course to center and to generate vertical space between the centered text and the surrounding text.

Concerning \centering it’s advisable to limit its scope by grouping. Inside a figure or table environment it’s already limited, but inside normal text you should use curly braces or \begingroup\centering … \endgroup:

{\centering Text
 
}

As you can see I set an empty line before closing the centered group. If I do not end the paragraph by a paragraph break or the line by \\ then the following text outside the group will be centered too. \centering is also defined by latex.ltx:

\def\centering{%
  \let\\\@centercr
  \rightskip\@flushglue\leftskip\@flushglue
  \parindent\z@\parfillskip\z@skip}

It’s using \leftskip and \rightskip to flush left and right.

This topic was discussed in the LaTeX Community Forum, on mrunix.de and on ubuntuusers.de .