Matrix multiplication
The multiplication of matrices is defined between two matrices only if the number of columns of the first matrix is the same as the number of rows of the second matrix, that is, you can multiply A_{mxn} and B_{pxq} only when n=p (the result it will be a mxq matrix).
For A_{mxn} and B_{nxp}, the product of both of them is A·B=C, where C is:
That is, c_{ij}=a_{i1}·b_{1j}+a_{i2}·b_{2j}+...+a_{in}·b_{nj}