® An m × n matrix is a square matrix if m = n ® A = [aij]m × m is a diagonal matrix if aij = 0, when i ≠ j ® A = [aij]n × n is a scalar matrix if aij = 0, when i ≠ j, aij = k, (k is some
constant), when i = j ® A = [aij]n × n is  an identity matrix, if aij = 1, when i = j, aij = 0, when i ≠ j