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On-off switch: How to remember what the line and circle mean. Think binary.

Filed in Encyclopedia ,Ideas
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Is our new glue gun powered on or off?

We were recently in a hotel in Berlin Germany and the room heater had the line-circle (| O) symbol on it and we couldn’t remember whether line means on and circle means off, or the line means off and circle means on.

Quality German engineering made the heater silent so there was no way to tell by listening.

The next day we got home to find our new glue gun had arrived. Same problem. And glue guns take a few minutes to heat up so that’s annoying (and possibly dangerous)

Wikipedia to the rescue.

Turns out it’s best not to think of it as a line and circle.

Think of it as a 1 and 0.

Recall your computer science, logic, electrical engineering, whatever classes:

0 is FALSE, low voltage, or off
1 is TRUE, high voltage, or on

Boom. Retained for life.

Want to remember which side of your rental car the gas cap is on?


  1. Daniel Reeves says:

    The little perks of a CS degree. It never occurred to me that it could be ambiguous! Which is also a good example of the illusion of transparency πŸ™‚

    October 31, 2016 @ 7:22 pm

  2. dan says:

    wouldn’t it be easier to just remember the 0 is like an O and means Off?

    September 22, 2017 @ 11:43 am

  3. dan says:

    Doesn’t “On” also begin with O?

    September 22, 2017 @ 11:56 am

  4. steve says:

    The Germans are wise enough to not use terms for ‘on’ and ‘off’ that may be confused with a zero. We messed up their language while moving it across the Channel.

    September 23, 2017 @ 12:26 pm

  5. Josh says:

    That would make perfect sense if it looked like a 1 and a 0 not a line and a circle. Because logically you would think a circle is a closed loop, or a circuit that would be on and line is an open circuit.

    December 3, 2017 @ 11:42 am

  6. Naomi says:

    I will never forget now. Thank you!!!

    May 14, 2018 @ 4:16 pm

  7. William Terry Hunefeld says:


    May 29, 2018 @ 9:18 am

  8. Dave says:

    A classic example of “the curse of knowledge” in full force.

    In a sense… it’s obviously obvious *if* everyone has the same background. But,.. wait… is that ever the case?

    My vote is that this design and it’s continued use (in consumer products) is a glorious failure.

    July 11, 2018 @ 4:19 pm

  9. Tom Henry Sr says:

    β€œ0” can always stand for OFF and use β€œI” to always mean IGNITION! Problem solved for life and all other conventions!

    September 25, 2018 @ 4:20 pm

  10. Graeme Harrison says:

    What’s wrong with a tick or cross. Simple-

    November 20, 2018 @ 4:39 pm

  11. Gary LA says:

    The current symbols are very nice and simple but I feel it should be a plus+ sign or light bulb symbol for “on” and maybe an O or Zero with a diagonal or horizontal line through it for “off” or “nothing”!?!?

    December 4, 2018 @ 4:52 pm

  12. Tom Winters says:

    so 0 is like zero current is going through, and 1 means a current of 1 is going through, therefore that means On. It always looked like a circle and a line to me. I have been swearing about these meaningless symbols for years.
    On keyboards, only the letter O is truly round. The number 0 is usually a bit of an oval shape. And the number 1 usually has a little tail at the top, even in European languages, and the letter I does not have a tail at the top. Therefore the O and I on these on/off switches do not register as the numbers 0 and 1 to people.

    January 5, 2019 @ 12:59 pm

  13. Ian says:

    Is it not easier just to remember ‘one is on’ that’s what I have worked out from this off the cuff

    April 29, 2019 @ 3:59 pm

  14. Bocephus says:

    I’m with Graeme. Would’ve been simpler to have used β€œ+” for on and β€œ-β€œ for off.

    June 15, 2019 @ 3:35 pm

  15. edward leon wier says:

    plus and minus are the superior symbols

    June 20, 2019 @ 6:24 pm

  16. Barry says:

    If it takes that much thought, it’s a fail. It should be a longer continuous line and 2 shorter broken lines “—” , “- -” (tough to “draw” here), but obviously connected and open…

    June 27, 2019 @ 3:24 pm

  17. J Arnott says:

    I think I seeing the – as being a wire that allows continuous flow of power, so by that interpretation the O represents a block of power.

    July 19, 2019 @ 11:10 am

  18. Robert Campbell says:

    If you must have a non language based symbol, I think + for on and – for off is much easier to remember and would work much better. I can’t imagine anyone of normal intelligence being unsure about pressing + for on.

    August 1, 2019 @ 11:29 am

  19. Margaret H says:

    Here are two options to help you remember…The number one has ‘on’ in the word one. So if you think of I and O, it’s a one and a zero. With ‘on’ in the word one, your switch is on.

    The other option. When current/power comes into your appliance from the wall, it has to get to the appliance to be able to work. It has to come in from the wall and go out to the appliance. A straight line is the best way to do this…thus, ‘I’. If the current comes in from the wall, and the appliance is off, the current will just sit there and go around and around without ever making your appliance heat up, light up, or work…thus, ‘O’ where the power never gets to come out and go to the appliance to make it work.

    Both are something I’ve used for years. Maybe it will help you.

    August 7, 2019 @ 2:46 pm

  20. Steve Williams says:

    Sheesh. Just write ON or OFF on the confounded thing. Is that so hard? Who voted to have sterile, incomprehensible electronic-ese all throughout our lives, anyway?

    August 10, 2019 @ 4:30 pm

  21. Anonymous says:

    How about thinking of O is “Off”

    August 11, 2019 @ 8:10 am

  22. HienN says:

    The Chinese symbol for 1 is β€œ-β€œ.

    So it wouldn’t make sense for them.

    β€œ+” and β€œ-β€œ makes more sense in my Opinion.

    Everyone who says β€œO” should be off.
    Do you not spell the word β€œOn” with an O?

    September 4, 2019 @ 11:14 am

  23. Anonymous says:

    Who’s every says the β€œ|” should represent 1 as in on. Have never taken Chinese.

    September 4, 2019 @ 11:19 am

  24. Rob says:

    How about just On & Off….
    It’s not like both words have a lot of letters so just use them either on the switch or either side of when the switch is mounted.

    September 20, 2019 @ 8:35 pm

  25. dug says:

    i work it as o for off i for ignition

    October 12, 2019 @ 9:17 pm

  26. Adam Garrett says:

    I would think that a “check mark” for on and an “X” for off would be pretty well universally understood. Check means yes to power, X means no.

    October 14, 2019 @ 12:02 pm

  27. Allen Lewis says:

    I have worked in the electronics industry for over 60 years. For at least 50 of those years they labeled on off switches with the word “OFF” if you wanted something off and “ON” if you wanted it on. Can’t go wrong there.

    December 10, 2019 @ 7:01 pm

  28. John Doe says:

    Why not simply have a solid small circle marking on the side that is mean to be on, and no marking on the off side? Language independent, hard to misinterpret, simple.

    January 16, 2020 @ 8:14 am

  29. David Ireland says:

    I hate the | & O markings! They tell me NOTHING! “Is the O ‘O’n or ‘O’ff?” We grew up with “On” and “Off”, why not just leave it that way?

    February 16, 2020 @ 4:20 pm

  30. Michael says:

    Looks like a pipe to me, circle is not blocked so water can flow through so β€˜o’ must be ON. Compacted, water can’t flow though so ’l’ must be off. (Except backwards).

    Not a good design.

    April 20, 2020 @ 7:41 am

  31. Niko says:

    The world has gotten so dumb it has resorted to using hieroglyphics, so many icons and symbols. I don’t know what they all mean.

    May 12, 2020 @ 11:29 am

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