Type E is the French answer to the problem of earthing, and it is also the standard plug of Belgium, Slovakia and Tunisia. At first glance it looks conventional enough: two round pins, each 4.8 millimetres thick, set on 19 millimetre centres, ready to push into a recessed circular socket. The surprise is where the earth connection lives. By the logic almost everyone else follows, the earth ought to be part of the plug. Here it is arranged backwards.

A male earth pin protrudes from the socket itself, standing up out of the round recess, and the plug carries a matching hole in its face that slides down over it. The socket reaches out to the plug rather than the other way around. That inverted arrangement has a quiet advantage. Because the earth pin sticks out of the wall and mates with a specific hole, it fixes how the plug can sit. There is only one way for the face to meet the pin, so the plug's orientation is locked the moment it goes in. This makes Type E inherently polarized, always seating line and neutral the same way round.

Its close relative, the German Schuko, shares the same round pins and the same recess but earths through side clips instead, and as a result the Schuko plug can go in either way up. Type E cannot, and that is by design. Rated at 16 amps on a 230 volt supply, it is a capable, everyday plug that hides one genuinely unusual idea behind an ordinary two-pin face. The earth is not something the plug brings to the socket. It is something the socket offers to the plug, and the whole orientation follows from that single reversal.