Answer: The Power Button

The international symbol for power buttons is a simple circle intersected at the top by a line. This symbol is the fourth evolution of the standard for power buttons since the 1940s. Early computer and electronic systems had toggle switches that were simply labeled On and Off on the respective sides of the toggle.

This standard was replaced fairly quickly in an effort to make switches universal across languages. On and Off were replaced with the numerals 1 and 0, a nod to the binary system, indicating the power was on (1) or the power was off (0). When toggle switches were replaced by push buttons, the symbol evolved again, this time to a circle with a line in the center indicating that the single button was capable of turning the machine on and off.

The introduction of lower-power states (such as the standby mode on modern computers) necessitated a fourth revision. The most recognizable iteration of the symbol, a circle broken by a vertical line, indicates that the button can power a machine on, off, or enter it into a power saving state. The photo here shows such a power button—with an on/off/standby design—on an older model MacBook Pro.