Dispersion along a cut across the nodal lines protected by mirror reflection symmetries carrying different topological invariants. Different colors represent opposite mirror eigenvalues. (a) An accidental nodal line which has ζ₀ = 0. (b) A protected nodal line carrying ζ₀ = 1. (c) Two nodal lines (one inner and one outer) carrying opposite invariants. (d) Two nodal lines carrying the same invariant.^[28]