Why the pmatrix environment get a higher parenthese than pmatrix command?

Question

I found the amsmath package's pmatrix environment have a bit difference with LaTeX's pmatrix command. You can see it from:

\documentclass{article}
% \usepackage{amsmath}
\linespread{1.3}
\begin{document}
$\pmatrix{A&B\cr C&D\cr}$
% $\begin{pmatrix}A&B\cr C&D\cr\end{pmatrix}$
\end{document}

and:

\documentclass{article}
\usepackage{amsmath}
\linespread{1.3}
\begin{document}
% $\pmatrix{A&B\cr C&D\cr}$
$\begin{pmatrix}A&B\cr C&D\cr\end{pmatrix}$
\end{document}

Is this a feature of amsmath?

Answer 1

This seems to be due to the \linespread{1.3}. I'm not sure if this is a feature of amsmath or a bug. Depends on your point of view, I suppose. Feel like a bug. Especially if you use an absurd \linespread factor (e.g., 5).

Answer 2

I never use \linespread, and if you look at the very fine document mathmode.pdf I think it will be difficult to find how to use \linespread. If you make some investigations on the web, you can find \linespread{1.3} \selectfont. I think if you really need to get some space, \renewcommand\arraystretch{1.3} is a better idea!

\documentclass{article}
\usepackage{amsmath}

\begin{document} 

\linespread{1.3}% do nothing !!! without \selectfont   

$\begin{pmatrix}A&B\cr C&D\cr\end{pmatrix}$  

\linespread{1.3} \selectfont 

$\begin{pmatrix}A&B\cr C&D\cr\end{pmatrix}$  

\renewcommand\arraystretch{1.3}
$\begin{pmatrix}A&B\cr C&D\cr\end{pmatrix}$ 

\end{document}