Vertically center rule on a page

Question

More precisely, I want to get a 18 × 26 cm² landscape box with a rule at the middle. Combining the crop package and this code I got:

\documentclass{article}
\usepackage[paperwidth=18cm,paperheight=26cm]{geometry}
\usepackage[a4,frame,center,noinfo]{crop}

\begin{document}%
\thispagestyle{empty}%
\topskip0pt
\baselineskip0pt
\vspace*{\fill}%
\noindent\makebox[\linewidth]{\rule{\paperwidth}{0.4pt}}%
\vspace*{\fill}%
\end{document}

But as you can see (I reduced the size to 5 × 2 so it's more visible), the rule isn't exactly halfway.

I assume this is due either to the box's margins or the rule's margins. How to work this out?

Answer 1

You have to specify a proper margin for the page. I have set a margin of 0in.

\documentclass{article}
\usepackage[paperwidth=18cm,paperheight=26cm,margin=0in]{geometry} %% specify equal margin
%\usepackage[a4,frame,center,noinfo]{crop} %% not needed
\usepackage{tikz}

\begin{document}%
\thispagestyle{empty}%
\topskip0pt
\baselineskip0pt
\vspace*{\fill}%
\noindent\makebox[\linewidth]{\rule{\paperwidth}{0.4pt}}%
\vspace*{\fill}%

%% to confirm the center
\begin{tikzpicture}[remember picture,overlay]
  \node[scale=3,text=red] at (current page.center) {X};
\end{tikzpicture}
\end{document}

The red X is the just to show the center. To be on safer side, you may set headheight, headsep and footskip also to be zero. You may do so in

\usepackage[paperwidth=18cm,paperheight=26cm,margin=0in,headheight=0in,headsep=0in,footskip=0in]{geometry}

A tikz solution:

\documentclass{article}
\usepackage[paperwidth=18cm,paperheight=26cm,margin=0in]{geometry} %% specify equal margin
%\usepackage[a4,frame,center,noinfo]{crop} %% not needed
\usepackage{tikz}

\begin{document}%
\begin{tikzpicture}[remember picture,overlay]
  \draw[olive, line width=8pt] (current page.south west) rectangle (current page.north east);
  \draw[red, line width=0.4pt](current page.west) -- (current page.east);
  \node[scale=3,text=red] at (current page.center) {X};   %%% uncomment just for demo
\end{tikzpicture}
\end{document}