One of two symbols composed of a Zero (Circle) and One (vertical line) overlaid on top of each other. Under the International Electrotechnical Commission (IEC) 60417 standard, Graphical symbols for use on equipment, 0 is the symbol for "off" and 1 is the symbol for "on." Combining these two symbols you get either the symbol for "on/off" (a circle with a vertical line inside of it) or "stand-by" (an incomplete circle with an opening at the top and a vertical line sticking out of the opening).