The question has been written in the title above.
Why not dimen=middle ?
only historical reason to allow a line, which didn't know anything about inner and outer to touch a circle/rectangle by default
\documentclass{article}
\usepackage{pstricks}
\begin{document}
\begin{pspicture}[showgrid,linewidth=5pt,
linecolor=red!60,strokeopacity=0.4](10,5)
\psframe[dimen=outer](0,1)(3,4)
\psframe[dimen=middle](4,1)(6,4)
\psframe[dimen=inner](7,1)(9,4)
\psline[linewidth=1pt,linecolor=black](0,4)(10,4)
\end{pspicture}
\begin{pspicture}[showgrid,linewidth=5pt,
linecolor=red!60,strokeopacity=0.4](10,5)
\pscircle[dimen=outer](2,1.5){1.5}
\pscircle[dimen=middle](5,1.5){1.5}
\pscircle[dimen=inner](8,1.5){1.5}
\psline[linewidth=1pt,linecolor=black](0,3)(10,3)
\end{pspicture}
\end{document}
\usepackage[showframe]{geometry}, which distance is equal to\textwidth: the horizontal distance between the frame rule's inner edges or the horizontal distance between the frame rule's outer edges? – Display Name Dec 24 '10 at 16:07\textwidth,\linewidth, etc. – Display Name Dec 25 '10 at 04:45\textwidth,\linewidth, etc. are absolute values and not in relation to inner/outer. This changes if you want to draw a square of width\textwidth, then you have to say how the frame of a given width should be drawn, inside, ouside or in the middle. but this is only a question of the used package, not a question of TeX. – Dec 25 '10 at 06:14