Obviously, due to the TRS, the Chern number is not a suitable parameter to describe the topological order of TRI topological insulator. Therefore, we defined “\(D\)” as the parity of the number of edge state pairs at an edge. For example, Fig.10(c) has 3 edge state Kramers pairs for any energy in the bandgap, the parity in this case is odd. The \(D\) is topological invariant. It was then realized that the number can be further defined as \(Z_2\) topological order.