Diagrams of nebular abundance ratios N/H vs. He/H (left panel) and N/O vs. O/H (right panel) for Galactic disc PNe. Blue symbols mark OC PNe (OC_cr – open triangles; OC_am – open squares; OC_am+cr – open reversed triangles). Red symbols mark CD PNe (DC_cr – open hexagons; DC_am+cr – open diamonds); black symbols mark CC PNe (CC_ar – filled hexagon; CC_al – filled circles; CC_ar+al – filled diamonds); green filled squares mark F type PNe. The horizontal/vertical lines mark the solar abundance values.