The physical background of resonance phenomena such as the “enhanced trans-mission” (through below cutoff holes) and the “optical activity” (polarization plane rotation) is shown by the eigenoscillation analysis of various objects with C₂ or C₄ symmetry in a square waveguide. Units of three kinds, namely waveguide bifurcations (plane junctions), double-slot or four-slot plane-chiral irises and composite conjugated plane-chiral irises in the square waveguide are considered. New compact units based on the fringing field interaction and aimed to rotate the polarization plane by 90° (in double-slot case) or by an arbitrary angle (in four-slot case) areproposed. The conclusions are completely applicable to double-periodical structures, as a possible base for development of metamaterials.