Understanding TikZ node placement

Question

I'm having a hard time understanding how TikZ uses space

\documentclass{article}
\usepackage{tikz}
 \usetikzlibrary{positioning}
\begin{document}
\tikzset{red/.style={fill=red,circle,inner sep=0pt}}
\tikzset{green/.style={fill=green,circle,inner sep=0pt}}
\begin{tikzpicture}[x=1cm,y=1cm]
  \draw[help lines,step=.2] (-2,-1) grid (-1,0);
 \draw[help lines,line width=.5pt,step=1] (-2,-1) grid (2,1);
 \foreach \x in {-2,-1}
 \node[anchor=north] at (\x,-1) {\x};
 \foreach \y in {-1,0}
 \node[anchor=east] at (-2,\y) {\y};
 \node [rectangle,inner xsep=20mm,inner ysep=10mm,line width=.5pt](fr){};

 \node [red,anchor=center] at (fr.south west){};
 \node [green,anchor=center] at (-2,-1)(sw){};
 \node [green,right=2mm of sw.center](r){};
\end{tikzpicture}
\end{document}

1) If you compile the above code you'll see that the red dot, which is placed with a relative coordinate, and the green one which is placed with numerical coordinates do not overlap perfectly. Why is that?

2) the node labeled r is not placed where it should be but a bit on the right, even if the distance is specified from sw.center. Why?

Answer 1

In the placing of the red node, the line width is being taken into account. For the second green dot, the node border is being used as reference, you should use on grid to ignore that.

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{positioning,calc}

\begin{document}

\tikzset{red/.style={fill=red,circle,inner sep=0pt}}
\tikzset{green/.style={fill=green,circle,inner sep=0pt}}

\begin{tikzpicture}[x=1cm,y=1cm]
  \draw[help lines,step=.2] (-2,-1) grid (-1,0);
  \draw[help lines,line width=.5pt,step=1] (-2,-1) grid (2,1);
  \foreach \x in {-2,-1}
    \node[anchor=north] at (\x,-1) {\x};
  \foreach \y in {-1,0}
    \node[anchor=east] at (-2,\y) {\y};
  \node [rectangle,inner xsep=20mm,inner ysep=10mm,line width=.5pt](fr){};

  \node [red,anchor=center] at ($(fr.south west)+(.25pt,.25pt)$) {};
  \node [green,anchor=center] at (-2,-1) (sw) {};
  \node [green,anchor=center,on grid,right=2mm of sw](r){};
\end{tikzpicture}
\end{document}