A vertical bar is used to visually set off the augmented column from the original matrix. Gaussian elimination works by adding scalar multiples of rows of the augmented matrix to rows below in such a way as to produce an upper triangular matrix, that is, a matrix for which all entries below the main diagonal are zero. If —1 is multiplied by the first row and then that scalar product is added to the second row, then the system in Eq. (5.6) becomes