# Why "p" in pmatrix?

I am using the command \begin{pmatrix} [...] \end{pmatrix}. I wonder why this command is not just called \begin{matrix} [...] \end{matrix} and why a "p" before "matrix".

• "p" for "parenthesis", the surrounding delimiter. One might surmise that bmatrix is for bracketed matrices. Jan 19 '16 at 15:16
• The matrix environment is the generic version -- it has no delimiters.
– user31729
Jan 19 '16 at 15:18
• B for braces, V for vertical bars and there are more. Jan 19 '16 at 15:19
• @StevenB.Segletes: You were the first who commented -- would you write the answer as an anthology on matrix delimiters? ;-)
– user31729
Jan 19 '16 at 15:37
• Related: Where is the \matrix command
– Werner
Jan 19 '16 at 16:36

The amsmath package provides a number of options. The leading letter before matrix indicates the delimiter that is used:
p for parens, b for brackets, v for verts, B for braces, V for double verts.
\documentclass{article}
$\begin{matrix} 1&2&3\\ 4&5&6\\ 7&8&9 \end{matrix} \quad \begin{pmatrix} 1&2&3\\ 4&5&6\\ 7&8&9 \end{pmatrix} \quad \begin{bmatrix} 1&2&3\\ 4&5&6\\ 7&8&9 \end{bmatrix} \quad \begin{vmatrix} 1&2&3\\ 4&5&6\\ 7&8&9 \end{vmatrix}$
$\begin{Bmatrix} 1&2&3\\ 4&5&6\\ 7&8&9 \end{Bmatrix} \quad \begin{Vmatrix} 1&2&3\\ 4&5&6\\ 7&8&9 \end{Vmatrix}$