Matrix and vertical lines in columns

Question

Can someone please tell me how I get vertical lines in the columns of a matrix like it is shown in the picture?

I started with ... = \begin{pmatrix} \lambda A' & \mu B' & \tau C' \end{pmatrix} ...

Answer 1

Use \mid as ordinary table entry.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[ M =
  \begin{pmatrix}
    \mid & \mid & \mid \\
    \lambda A' & \mu B' & \tau C' \\
    \mid & \mid & \mid
  \end{pmatrix}
\]
\end{document}

Answer 2

You could use a \vline

\documentclass{article}
\usepackage{amsmath}
\begin{document}

$ M = \begin{pmatrix}
\vline & \vline & \vline \\
\lambda A' & \mu B' & \tau C' \\
\vline & \vline & \vline
\end{pmatrix}$
\end{document}