Reverse polarity protection for plug-in connectors comprising two intermateable plug-in connector parts (100, 200), wherein the one plug-in connector part (100) comprises a first coding element and the other plug-in connector part (200) comprises a second coding element and wherein the two coding elements are matched to each other in such a manner that when the plug-in connector parts (100, 200) are arranged correctly they make a plug-in connection possible and that when the plug-in connector parts (100, 200) are not arranged correctly prevents a plug-in connection, characterized in that the one coding element is a groove (110) extending in the mating direction and having a trapezoid-shaped cross-section, and that the other coding element is a coding rib (210) that is extending in the mating direction and that is formed in a U-shaped manner with elastically bendable walls (212, 213) that are arranged in a U-shaped configuration.