The locus of points on the solar surface where the radial magnetic field vanishes. Inversion lines separate regions of opposing polarity and are often superposed by thin, dark filaments, which can be used as tracers. Inside active regions, the areas close to and along inversion lines are preferred places of flare occurrence. Filament channels, plage corridors, arch filament systems, and fibril patterns surrounding active regions can be used to infer the positions of inversion lines.