Japan and the U.S. adopted standardized electricity supply systems of 100V and 120V respectively, which differs from some other countries that went for a higher line voltage for commercial or industrial reasons. While both power systems are inherently similar, the two voltages adopted by the U.S. and Japan share a few characteristic differences.
While a distribution transformer used for 100 to 120V generation will certainly have two or more primary windings wired in parallel, the 100V capacity is usually accommodated by an additional primary winding, a tap on the 120V primaries or even a boost/buck winding in series with the 120V primary winding. In layman's terms, you can regulate voltage at the output by selecting only a certain number of turns in the transformer's primary winding. This configuration is usually at the service entrance, where the power source connects with the supplied building.
Most 100V sockets and plugs used in Japan are non-polarized, which means you can connect the plugs in any orientation with the live and neutral poles matched arbitrarily. On the other hand, 120V sockets in the U.S. require polarization, which means the live and neutral connectors of the outlet must connect to the counterpart poles in the appliance. This means that a North American 120V plug will not connect to Japanese 100V sockets without an adapter.
Most available 120V electrical outlets are either type A and B, but they also come in G and I form for foreign markets such as Guatemala. 100V electrical outlets, on the other hand, are nearly always manufactured in type A form, also called the National Electrical Manufacturers' Association 1-15 type. The type B form, commonly known as NEMA 5-15, is less common.
Power outlets rated at 100V are analogous to 120V power sockets in most respects, and most appliances function properly both without needing an adapter. This rule doesn't apply in some specific categories like heating appliances, such as hair dryers or water heaters. A heat-generating device from Japan will get 20 percent hotter when used in the U.S., potentially resulting in damage to the device or an accident that could injure the user. Connecting such appliances through a small adapter involving a step-up or a step-down transformer is mandated to prevent such mishaps.