TITLE: How to denote matrix concatenation?
QUESTION [20 upvotes]: Trivial question: Is there any standard notation for the concatenation of two or more matrices?
Example:
$$A = \left(\begin{array}[c c] 
- a_1 & a_2\\
a_3 & a_4
 \end{array}\right),$$
$$B = \left(\begin{array}[c c] 
- b_1 & b_2\\
b_3 & b_4
 \end{array}\right),$$
Then the concatenation (by rows) of $A$ and $B$ is:
$$C = \left(\begin{array}[c c] 
- a_1 & a_2\\
a_3 & a_4 \\
 b_1 & b_2\\
b_3 & b_4
 \end{array}\right).$$
I just want to know if there is a standard notation for this operation.

REPLY [4 votes]: In the example given,
$$
C = \begin{bmatrix} A \\ B \end{bmatrix}
$$
is a standard notation. This is a particular example of "block notation".