A circle broken by a vertical line, and the whole world reads it as power. The IEC power symbol is one of the most universal marks in the built environment, stamped on switches, screens and buttons everywhere to mean on and off, or to signal that pressing here will shut something down. It is a hardware-grade metaphor, born on physical devices and carrying that heritage with it wherever it goes. The line that pierces the circle gives it a decisive, mechanical feel, the look of something that latches and clicks rather than merely closes.

That heritage is exactly why the symbol asks to be used carefully in software. It descends from the physical act of powering a machine, and it works best when it still means precisely that. Reserve it for genuinely powering something on or off: a device that will actually turn off, a session that will truly end, a service that will start or stop. In those places the symbol reads true, its meaning lining up with what the button does. Where it goes wrong is in the small, everyday moment of closing a dialog. A power symbol on a close button promises far more than the button delivers, suggesting shutdown when all that happens is a panel sliding away.

The metaphor overshoots, and the user is left with a mark that means too much for the job. The discipline, then, is to match the weight of the symbol to the weight of the action. This glyph is heavy. It speaks of current and cut-off, of things going dark. Given to a genuine power control it is perfect, instantly understood, universally known. Handed to a trivial dismissal it is simply the wrong tool, a strong word used for a small thing. The circle and its line have earned their meaning; the least an interface can do is honour it.