The earliest civil power supply was direct current (USA). According to the technical level at that time, it was set to 1 10V for safety reasons (most people can separate this voltage by themselves, and a few people can touch it directly without feeling it. But it is not safe, people will still die).

Later, alternating current came out and the United States also used this voltage. However, European countries, led by Germany, think that this voltage consumes too much material (the lower the voltage, the thicker the copper wire is needed), so it is doubled to 220V V V. At this time, the United States can't change it because electricity has become popular. So there are two kinds of power sources in the world: AC 100 V ~ 130 V and AC 220 ~ 240 v.

China's power grid was brought in by foreigners. At first, it was a small LAN with various voltages. After liberation, it was unified as 1 10V and 220V. Later, after studying the Soviet Union, 220V accounted for the majority, and/kloc-0 was basically unified into 220V in the 1960s.

Standards are always set by the people who invented them first. At that time, Microsoft made a software called Windows. Now, as soon as I use the computer, it is said that I open a window. Actually, you can call a door, a box or a room, but is it easy to change?