I've asked that question somewhere else and was told that Japan had adopted metric at the same time and took 100 as a nominal voltage. Japanese households really didn't get electrified until the 1920s.
Their household plugs at first were lamp-base and then flat blade, same as the US.
I've visited Tokyo a few times. There's a big DIY chain store called "Tokyu Hands", and they have an electrical department.
I remember seeing a 200V plug. It's like a NEMA 2-15P, no ground pin. So just like the US, I guess they add the two service 100V hot wires to get the 200V.
Btw, here's a ref chart for NEMA plugs:
http://www.leviton.com/sections/techsupp/nema.htm I brought back a few Japanese household receptacles from Tokyu Hands. The faceplate just snaps on and the connections are push-in. Of course it's ungrounded NEMA 1-15R, white lead neutral. The rating is 125V 15A.
The only place I saw a grounded receptacle was at a donut shop, probably where 100V voltage grounded appliances are used.
I also bought a voltmeter and measured the line voltage and it was 105.
Also, there's a place in San Francisco's Japantown that sells some 100V appliances and stepdown wall transformers to bring 120 to 100V.
Gene--
