Notation for juxtaposition operation on matrices Does there exist a fairly standard notation for horizontal and vertical juxtaposition operations on matrices?  (Vertical juxatposition can be called "stacking.")
For example, juxtaposing horizontally a matrix of the size $m\times n_1$ with a matrix of the size $m\times n_2$, one obtains a matrix of the size $m\times(n_1 + n_2)$.  Stacking vertically a matrix $m_1\times n$ with a matrix $m_2\times n$, one obtains a matrix $(m_1 + m_2)\times n$.
I've seen the notation
$$
A = [a_1|a_2|\dotsb|a_n]
$$
for a matrix $A$ with columns $a_1,a_2,\dotsc,a_n$.  It looks a bit ad hoc, unless we define $|$ as the horizontal juxtaposition operation that can be applied to any pair of matrices with the same number of rows.  A notation for vertical juxtaposition is also needed.
Such notation would be quite useful for writing matrices defined by blocks or decomposing matrices into blocks (into rows or columns in particular).

Just draw a matrix of matrices:
$$M=\begin{pmatrix}A&B\\C&D\end{pmatrix}$$
