The evolution of the On/Off power switch symbol

We all know the symbol with a vertical line in a circle: it identifies the On/Off power switch. It occurred to me that this familiar symbol is evolving in a bizarre fashion.On Off Power Switches

Originally, switches had a lever or slider that could move to either of two physical positions. In those days the switch was marked with the word POWER and its positions with ON and OFF. Then, as switches became smaller and more globalized, the two words were replaced with 1 and 0, as seen even today on many rocker switches.

And then the ubiquity of microprocessors made it more economic to do everything with momentary pushbutton switches; the computer inside could take care of figuring whether you meant ON or OFF. And so, the button now needed an icon that conveys both options; I surmise that is when the familiar “1-inside-a-0” symbol came into existence (if you know otherwise do share in the comments!) This round icon fit nicely on round buttons, and became ubiquitous.

OnOff power switches

But then we start to see the form shown in the two photos above right: a bastardized version combining the 1-in-a-circle with a 1 in the same symbol. This makes no sense at all – the correct representation would have been 1/0, for On slash Off. Instead we get On slash OnOff. Sloppy thinking…

Such erroneous contractions are often seen in spoken language – as in “IT technology”, which expands to “information technology technology” (there’s even a company by that name, and its slogan, amusingly, is “We make sense of IT“). But now we see the same error invading the more compact space of visual symbols…

    70 Responses to “The evolution of the On/Off power switch symbol”


    1. 1 steve

      I believe the intention of the symbols changing on the switch to a “1” & a “0” is directly related to our society breaking into a new technological age, which has only been possible through computers, and a computers’ native language is binary. (Zeroes ones).

    2. 2 Philip

      The cirlce and line stand for 0 and 1 in binary. The switch we recognize as “on-off” was originally meant to indicate a button that put a device into a low power or sleep state. A true on-off symbol would be a closed 0 with a 1 inside it. So those buttons are probably correctly showing an on-sleep switch.

    3. 3 Philip
    4. 4 maxweber

      If you’ve designed UI then you know why its one button… the original icons were useless. Does O mean Off? Does O mean light? Does l mean screen is collapsing to off? Does it mean a switch connection? Nobody knows. That’s why the toggle switch worked. People have no idea what the l and O icons mean. That’s just some bad ui design for techies. design by committee. I’ll bet if you ask 10 people on the street what O and l mean then you’ll not get even 70% getting it right. So, the combined symbol now means “change power state”. And user still have to guess what that means WRT hibernate/sleep (platform prompts them often).

    5. 5 Jul

      Great post! But I have a weird question about this. Why use the words “ON” and “OFF”? Going back to an era before the traditional switch, maybe even as far back as kerosene lamps, when you ignited something, was it always called ON and so forth? Why call it an ON/OFF switch and not a START/STOP or IGNITE/EXTINGUISH?

    6. 6 Fishy

      Oh. I always thought this means power (masculine) and this is a sketch of male and female organs in an intercourse, just like male and female sockets. Thanks for valuable info.

    7. 7 Nathan Zeldes

      Good question, Jul! I admit I have no idea!…

    8. 8 Dennis

      Ok, now tell me why one side of a plug is larger and only fits into the outlet one way. Years ago this wasn’t the case. Has anyone EVER in their lifetime plugged something in the right way the first time? What about when you have to plug something in that is awkward and difficult to see? You never know which way to hold the plug. What difference does it make anyway?

    9. 9 Nathan Zeldes

      @dennis, this is off topic, but I believe the polarized plug is meant to control which of the wires has the “Live” side of the mains and which the neutral – a potential matter of life and death in case of a leak to the casing of the appliance (not critical in double insulated appliances, though).

    10. 10 satish

      O & I represent simply IN & OUT
      what the whole ruckus about!
      simply derived from the toilet door or just any thing which shows the status (Position) of the commodity (which in case here it is power / current). O – Current is Out of the device. I – Current is in the device!

    11. 11 Gary Shockley

      “O” refers to an “open circuit” in which current cannot pass through, so the device is OFF.

      “1” is not a “one”, but is a vertical bar symbolizing a “closed circuit”, in which current will pass through, so the device is ON.

      An electrical circuit is either Open or Closed. Think of a light switch. When the switch is open, current cannot pass through, so the switch is off. When the switch is closed and metal contacts touch, then the circuit is closed and current is running through it.

    12. 12 John

      This is just ridiculous. Why should people have to try to remember or come up with some memory aid to figure out which of the symbols means on and which means off. Just use your words and put “On” and “Off” on the switch! Sheesh!

    13. 13 Belinda Klatte

      I’ve always wondered about the origin of these symbols hence ending up here, after much searching but looking for the letters I & O not numbers. My query re numbers is, fine with the written number being a single line but on a keyboard I don’t think I’ve ever seen the number 1 typed as a single line so how did this come to be?? Numbers and letters aside though, with vertical switches up is always off and down always on – with horizontal ones left is always off and right on. The only exception being with multiple vertical switches to the same power source such as household lights – as far as I’ve seen anyway. A great thread, it was very interesting!

    14. 14 Omer Kahoot

      Great informative post.
      But one thing that confuses me is that many people in the comments have said that O stands for the open circuit and 1 is basically a straight line that represents a closed circuit.
      But how do we know, which notation came first?
      Was it 0 for open circuit or 0 for off switch.

      Maybe, 0 was considered off switch and when they decided to give a symbol for an open circuit, they gave 0 since no current is flowing in both cases, or was it the opposite scenario, or were both given at the same time.

      Thanks.
      All in all, a very interesting thread.

    15. 15 bill benfield

      What a farce!!!!!! 1 means on and O means off. Why can’t power tool manufacturers simple put “on and Off”? Nowhere in the manufacturer’s manuals do they tell you that “1” means “on” and “O” means “off”. Who is the dummy here?

    16. 16 Dai tree

      Right, I’m old enough to remember switches marked I and O for starters. I’ve been thinking about what ( as has already been pointed out) is the ‘stand by’ switch, symbolically speaking, I get the binary argument and the more traditional electrical current perspective ( and the ‘seen on a UFO’ argument). I love ikea instructions – the little symbols used to transcend language (to save on printing costs in the globalised consumer market) – a simple visual prompt.
      This power button thing ? Think of the line as a finger and the circle (or broken circle) as a button – the finger is pushing the button.
      Push the button ? Go on, as someone pointed out above , you’ll soon work out if it’s on or not

    17. 17 Chris

      It isn’t a “1” or an “O” it’s a line that indicates closed circuit or a circle for closed circuit or what “on/off” looks like on a wiring diagram or schematic.

    18. 18 Texas Johnny

      What the fuss is all about is that I have some Electrical Workers that can not figure out a rocker switch with the Power sign indicate a simple no brainer except my electricians do not pay attention to little problems of which become big problems when the systems fail to light such in solar power arrays.
      Below the Rocker Switch is marked POWER, and then the Rocker switch indicates a 1 below and a O above. A simple test of the switch with an OHM Meter will indicate ON or OFF when the switch is toggled. Well thank for genuises

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