All of Europe, Africa, Asia, Australia, New Zealand and most of South America use a supply that is within 6% of 230 V, whereas Japan, Taiwan, North America and some parts of northern South America use a voltage between 100 and 127 V. The 230 V standard has become the most widespread so that standard 230 V equipment can be used in most parts of the world with the aid of an adapter or a change to the equipment's connection plug for the specific country. In the UK electricity is supplied at 230 V +10% - 6% (BS 7671 Appendix 2 section 14. )
The choice of utilization voltage is due more to historical reasons than optimization of the distribution system—once a voltage is in use and equipment using this voltage is widespread, changing voltage is a drastic and expensive measure. A 230 V distribution system will use less conductor material to deliver a given amount of power because the current, and consequently the resistive loss, is lower than for a 120 V system.
During the development of commercial electric power systems in the late 19th and early 20th centuries, many different frequencies (and voltages) had been used. Large investment in equipment at one frequency made standardization a slow process. However, as of the turn of the 21st century, places that now use the 50 Hz frequency tend to use 220-240 V, and those that now use 60 Hz tend to use 100-120 V. Both frequencies coexist today (Japan uses both) with no great technical reason to prefer one over the other and no apparent desire for complete worldwide standardization.