A common use of the voltage converter is for a device that 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 auto transformer 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, hairdryers, 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.