Multiplication of Matrices. Matrix Multiplication : 2×2 And 3×3 The definition of matrix multiplication uses a row-by-column multiplication, where the entry in the ith row and jth column of the product AB is obtained by multiplying the entries in the ith row of A by the corresponding entries in the jth column of B and then adding the results. So, for the product of two matrices to be defined, the number of columns of the first matrix must equal the number of rows of the second matrix. #MATRIX ;