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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 🙂

    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:

    “0” can always stand for OFF and use “I” 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 “+” for on and “-“ 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

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