A torus T² wrapping the nodal line γ₁ in the Brillouin zone. T² is thin enough so that it does not intersect any other band degeneracies. Red lines (γ₁ and γ₆) are lines of touching between the conduction and valence bands. Orange (γ₃ and γ₄) and green (γ₂ and γ₅) lines are lines of touching between the first and the second topmost occupied bands and between the second and the third topmost occupied bands, respectively. Only the linking between γ₁ and γ₃ is protected as it generates the nontrivial second Stiefel–Whitney number on T², whereas the linking between γ₁ and γ₅ is not protected [See Eq. (33)]. Figures are adopted from the supplemental materials in Ref. [7].