Travel adapter allows appliances made for the mains voltage of one geographical region to operate in an area with different voltage. Such a device may be called a voltage converter, power converter, travel adapter, etc. Most single phase alternating-current electrical outlets in the world supply power at 210–240 V or at 100–120 V. A transformer or autotransformer can be used; (auto)transformers are inherently reversible, so the same transformer can be used to step the voltage up, or step it down by the same ratio. Lighter and smaller devices can be made using electronic circuitry; reducing the voltage electronically is simpler and cheaper than increasing it. Small, inexpensive, travel adapters suitable for low-power devices such as electric shavers, but not, say, hairdriers, are available; travel adapters usually include plug-end adapters for the different standards used in different countries. A transformer would be used for higher power.
Transformers do not change the frequency of electricity; in many regions with 100–120 V, electricity is supplied at 60 Hz, and 210–240 V regions tend to use 50 Hz. This may affect operation of devices which depend on mains frequency (some audio turntables and mains-only electric clocks, etc., although modern equipment is less likely to depend upon mains frequency). Equipment with high-powered motors or internal transformers designed to operate at 60 Hz may overheat at 50 Hz even if the voltage supplied is correct.
Most mains-powered electrical equipment, though it may specify a single nominal voltage, actually has a range of tolerance above and below that point. Thus, devices usually can be used on either any voltage from approx. 100 to 120 V, or any voltage from approx. 210 to 240 V. In such cases, voltage converters need only be specified to convert any voltage within one range, to a voltage within the other, rather than separate converters being needed for all possible pairs of nominal voltages (110–220, 117–220, 110–230, etc.)