Matrix Multiplication
For a matrix multiplication to be valid, you need two vectors $A$ and $B$ such that $A$ is of size $m \times n$ and $B$ of size $n \times p$ and the multiplication must occur in the order $AB$, and results in a matrix of size $m \times p$. In order to apply the multiplication you use the formula below:
$$ AB = \begin{bmatrix} Ab_1 & Ab_2 & Ab_3 & … & Ab_p \end{bmatrix}
$$
Matrix multiplication is equivalent to applying two Linear Transformations one after another, this is why the order of multiplication matters