Sometimes a shape, as we see it, is more than just a group of connected lines. Universally, symbols make much more sense to people regardless of language barriers. One such example that has been around for quite long, is of the iconic symbol that has a vertical line in a circle – the On/Off power switch.
Originally, switches had a lever that could move in either direction with the two positions marked as On and Off. With time all components continued to reduce in size, hence the switches became smaller, which resulted in the two words being replaced with 1 and 0(typically without serifs). Then came the advent of microprocessors and the symbol had to be further reduced in size.
The “1-inside-a-0” symbol was result of the logical evolution in user interface design and an economic way to convey both ON and OFF. The circular part of the button represents the binary number 0 (off state) and the vertical line represents the binary number 1 (on state). This symbol was also independent of the dependence on English language; which was the case earlier when the power switches had On and Off written on them. In 1973, the International Electrochemical Commission first included the symbol to their set of graphics symbols for use on electronics.
The toggle or rocker buttons still have the line outside the circle which means switching the button on either side will turn the power ON/OFF completely.
The power symbol that is seen more frequently is withthe line half inside the circle (generally seen on push-type buttons). This means the power button will not cut off the power completely but put it in standby mode. And hence, technically, it is known as the standby symbol.