Why doesn't the column align properly while the row does in this TikZ picture?

Question

For some reason, the column doesn't align properly with matrix B. And I can't figure out why. I hate having to resort to using yshift this many millimeter this and this many millimeter for this.

\documentclass[11pt]{standalone}

\usepackage{tikz}

\usetikzlibrary{shapes}
\usetikzlibrary{arrows}
\usetikzlibrary{positioning}
\usetikzlibrary{calc}

\definecolor{shade1}{rgb}{0.9, 0.9, 0.9}
\definecolor{shade2}{rgb}{0.8, 0.8, 0.8}
\definecolor{shade3}{rgb}{0.65, 0.65, 0.65}
\definecolor{shade4}{rgb}{0.45, 0.45, 0.45}

\begin{document}

\begingroup
\newlength{\matrixSize}
\setlength{\matrixSize}{2cm}
\newlength{\matrixSep}
\setlength{\matrixSep}{4mm}
\begin{tikzpicture}[%
    auto,
    box/.style={%
      draw,
      rectangle,
    },
    matrix/.style={%
      box,
      fill=shade2,
      minimum size=2cm,
      inner sep=0pt,
      node distance=\matrixSep,
    },
    cell/.style={%
      box,
      fill=shade3,
      minimum size=0.1\matrixSize,
      inner sep=0pt,
    },
  ]

  % Matrices
  \node [matrix]             (C) {C};
  \node [matrix, left=of C]  (A) {A};
  \node [matrix, above=of C] (B) {B};

  % Cell to calculate in C
  \node [cell] at ([xshift=-0.2\matrixSize, yshift=0.2\matrixSize]
    C.south east) (cell) {};

  % Row from A
  \path let \p1 = (cell),
            \p2 = (A.base),
            \p3 = (A.west),
            \p4 = (A.east)
         in
           node [%
             cell,
             minimum width=\x4-\x3-\pgflinewidth,
           ] at ([yshift=\y1-\y2] A.base) (row) {};

  % Column from B
  \path let \p1 = (cell),
            \p2 = (B.base),
            \p3 = (B.south),
            \p4 = (B.north)
         in
           node [%
             cell,
             minimum height=\y4-\y3-\pgflinewidth,
           ] at ([xshift=\x1-\x2] B.base) (col) {};
\end{tikzpicture}
\endgroup

\end{document}

Answer 1

You need to use the B.center anchor instead of the B.base one; the latter is located a little lower than the former. In the following code I show the difference, using a red circle for B.center and a blue circle for B.base:

\documentclass[11pt]{standalone}

\usepackage{tikz}

\usetikzlibrary{shapes}
\usetikzlibrary{arrows}
\usetikzlibrary{positioning}
\usetikzlibrary{calc}

\definecolor{shade1}{rgb}{0.9, 0.9, 0.9}
\definecolor{shade2}{rgb}{0.8, 0.8, 0.8}
\definecolor{shade3}{rgb}{0.65, 0.65, 0.65}
\definecolor{shade4}{rgb}{0.45, 0.45, 0.45}

\begin{document}

\begingroup
\newlength{\matrixSize}
\setlength{\matrixSize}{2cm}
\newlength{\matrixSep}
\setlength{\matrixSep}{4mm}
\begin{tikzpicture}[%
    auto,
    box/.style={%
      draw,
      rectangle,
    },
    matrix/.style={%
      box,
      fill=shade2,
      minimum size=2cm,
      inner sep=0pt,
      node distance=\matrixSep,
    },
    cell/.style={%
      box,
      fill=shade3,
      minimum size=0.1\matrixSize,
      inner sep=0pt,
    },
  ]

  % Matrices
  \node [matrix]             (C) {C};
  \node [matrix, left=of C]  (A) {A};
  \node [matrix, above=of C] (B) {B};

  % Cell to calculate in C
  \node [cell] at ([xshift=-0.2\matrixSize, yshift=0.2\matrixSize]
    C.south east) (cell) {};

  % Row from A
  \path let \p1 = (cell),
            \p2 = (A.base),
            \p3 = (A.west),
            \p4 = (A.east)
         in
           node [%
             cell,
             minimum width=\x4-\x3-\pgflinewidth,
           ] at ([yshift=\y1-\y2] A.base) (row) {};

  % Column from B
  \path let \p1 = (cell),
            \p2 = (B.base),
            \p3 = (B.south),
            \p4 = (B.north)
         in
           node [%
             cell,
             minimum height=\y4-\y3-\pgflinewidth,
           ] at ([xshift=\x1-\x2] B.center) (col) {};

\node[circle,red, draw] at (B.center) {};
\node[circle,blue, draw] at (B.base) {};
\end{tikzpicture}
\endgroup

\end{document}

A little additional explanation: if the text label field of a node with rectangle shape is empty, the .base and .center anchors will be equal; however, adding a text label produces a vertical shifting of the .base anchor, as can be seen with the following code:

\documentclass[11pt]{standalone}
\usepackage{tikz}

\begin{document}

\begin{tikzpicture}[node distance=1cm and 1cm]
  \node[name=t,shape=rectangle] {};
  \foreach \anchor/\placement in {center/above, base/below}
    \draw[shift=(t.\anchor)] plot[mark=x] coordinates{(0,0)} 
      node[\placement] {\scriptsize\texttt{(t.\anchor)}};
  \node[name=s,shape=rectangle,xshift=3cm] {\Huge A};
  \foreach \anchor/\placement in {center/above,base/below}
    \draw[shift=(s.\anchor)] plot[mark=x] coordinates{(0,0)}
      node[\placement] {\scriptsize\texttt{(s.\anchor)}};
\end{tikzpicture}

\end{document}