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Suppose you have graphics which may or may not be able to fit within the width of the page; if they will fit, you want to set them at their natural size, but otherwise you want to scale the whole picture so that it fits within the page width.

You do this by delving into the innards of the graphics package (which of course needs a little LaTeX internals programming):

\makeatletter
\def\maxwidth{%
  \ifdim\Gin@nat@width>\linewidth
    \linewidth
  \else
    \Gin@nat@width
  \fi
}
\makeatother

This defines a “variable” width which has the properties you want. Replace \linewidth if you have a different constraint on the width of the graphic.

Use the command as follows:

\includegraphics[width=\maxwidth]{figure}