Skill: Identify rows and columns of a matrix

Rows run horizontally from left to right, and columns run vertically from top to bottom. To describe a matrix’s size, count the rows first and the columns second; write the size as $m \times n$.

For example, consider

$$A= \begin{bmatrix} 2 & 5 & 1\\ 0 & 4 & 7 \end{bmatrix}.$$

1. Count the horizontal rows: there are $2$.
2. Count the vertical columns: there are $3$.
3. Write rows first, then columns: the size of $A$ is $2 \times 3$.

So this matrix has $2$ rows and $3$ columns.