France and Belgium have a standard socket which is not compatible with the CEE 7 socket that is standard in Germany and other continental European countries. The reason for incompatibility its that grounding in the French/Belgian socket is accomplished with a round male pin permanently mounted in the socket. The CEE 7/7 plug bridges the differences between the two types of sockets. It has grounding clips on both sides to mate with the CEE 7 socket and a female contact to accept the grounding pin of the French/Belgian socket. See the illustration below.
Note: The CEE 7/7 plug is polarized when used in the French and Belgian electrical system.There are two types of polarization: physical and electrical. The illustration above demonstrates physical polarization, i.e. the ability to allow or stop a connection being made with the plug in one or more positions. Electrical polarization occurs when each line or connection of the supply is connected to a specific contact on the plug or socket, which is dictated by either a specific standard, or common practice. France and Belgium have no standard for wiring connectors, therefore, electrical polarization is non-existent.