Mains electricity is the general-purpose alternating-current (AC) electric power supply. In the US, electric power is referred to by several names including household power, household electricity, house current, powerline, domestic power, wall power, line power, AC power, city power, street power, and grid power.The two principal properties of the electric power supply, voltage and frequency, differ between regions. A voltage of (nominally) 230 V and a frequency of 50 Hz is used in Europe, most of Asia, most of South America and Australia. In North America, the most common combination is 120 V and a frequency of 60 Hz. Other voltages exist, and some countries may have, for example, 230 V but 60 Hz. This is a concern to travelers, since portable appliances designed for one voltage and frequency combination may not operate or may be destroyed by another.The use of different plugs and sockets in different regions provides some protection from accidental use of appliances with incompatible voltage and frequency requirements.