# Vertical bar in matrix appearing twice

$$\left( \begin{array}{c|c} \Huge{a} & \Huge{b} \\ \hline \Huge{c} & \Huge{d} \end{array} \right) \quad \left( \begin{array}{c|c} \Huge{A_{11}} & & \Huge{A_{12}} \\ \hline \Huge{0} & & \Huge{A_{22}} \end{array} \right)$$ Exactly why does that vertical line appear only once, as it should, on the left, and twice, as it should not, on the right?

MathJax code:

\left(
\begin{array}{c|c}
\Huge{a} & \Huge{b} \\
\hline
\Huge{c} & \Huge{d}
\end{array}
\right)
\left(
\begin{array}{c|c}
\Huge{A_{11}} & & \Huge{A_{12}} \\
\hline
\Huge{0} & & \Huge{A_{22}}
\end{array}
\right)

• It's really an error: "Extra alignment tab has been changed to \cr." (I know the text sounds like a warning, but it's counted as an error.) It does still produce an output, but by bumping the $A_{12}$ to the first entry of a new row, rather than by adding the third column. – mdp Apr 25 '14 at 15:46