eso-pic: different images on even and odd pages

Question

What I want

Graph paper as a background on every page with one margin:

• on odd pages the margin should be on the right
• on even pages the margin should be on the left

What I got

So far I managed to get a margin on the right side of every page.

MWE

\documentclass{article}

\usepackage{tikz}

\usepackage{eso-pic}

\AddToShipoutPicture{%
  \begin{tikzpicture}[remember picture, overlay]

  \tikzset{normal lines/.style={black!20, very thin}} 
  \tikzset{margin lines/.style={black!20, thick}} 

  \node at (current page.south west){
    \begin{tikzpicture}[remember picture, overlay]

      \draw[style=normal lines,step=0.5cm] (0,0) grid +(210mm,297mm); 

      \draw[style=margin lines] (180mm,0)--(180mm,297mm); % odd page

    \end{tikzpicture}
  };
  \end{tikzpicture}
}

\usepackage{blindtext}

\begin{document}

\blindtext[20]

\end{document}

Answer 1

Is an \ifodd\value{page} <odd page> \else <even page> \fi conditional:

\ifodd\value{page}
  \draw[style=margin lines] (180mm,0)--(180mm,297mm); % odd page
\else
  \draw[style=margin lines] (30mm,0)--(30mm,297); % even page
\fi