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…

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


    1. 1 Dominic Tramontana

      Awesome post, Nathan!

    2. 2 Nick

      i believe the bastardized version of the on off switch actually is also a reset button. So rather than make another symbol for reset the decided to go with on /onoff i think it would make more sense to put on/onoff/off.

    3. 3 Jeff

      I used to think the 1 and the 0 symbols were retarded because nowhere are you told what either one meant. Now that i know that the 1 is a 1 and not just a line ,its not so retarded after all.
      Thank you for setting me straight on these alien symbols.

    4. 4 Robert

      Here is an explanation from IEC and ISO.

      Related to the 0 with a broken ring and a 1 through it. It is soft off.
      >>
      For use on a power switch or button if the off state is soft-off, is variable, is not
      known, or the distinction from hard-off is not important. Also for use with a
      power indicator, or as the icon for the power control panel.
      >>

    5. 5 Arol

      I hope this help us to understand.

      http://en.wikipedia.org/wiki/Power_symbol

    6. 6 Peter-John Taylor

      Excellent, informative and timely post Nathan!

      We’d be very interested to know your thoughts on the matter of poor power switch design and function in general and especially what you think about “Bad design on a UPS” posted by Saaed at
      http://onproductmanagement.wordpress.com/category/business-topics/product-management/

      He writes “Last year, I got my hands on a pretty good UPS for my home computer network. It’s from APC. The model # is the XS1200. It’s a good UPS [which] can take 8 devices plugged into it. Six are managed, two are surge protected only. I’ve got all my critical devices plugged into it including my desktop computer, monitor, cable modem, router, some kind of phone/cable switching device (I get my home phone service through my cable provider), and a couple of other things.

      Now, this is a great device except for one fatal flaw. The round circle on the front is the on/off switch for the UPS. It’s also a very sensitive switch. It doesn’t take more than the soft fingers of a 2 year old child to turn it off. Yes, to turn the whole darn UPS and all 8 devices connected to it off! … I’m sure you can picture what has happened more than once.”

      See APC’s reply at:
      http://www.apc-forums.com/thread.jspa?messageID=8068&tstart=0

    7. 7 john f hubert

      I am a dummy. Is ‘1’ depressed on the rocker switch on or off?

    8. 8 Nathan Zeldes

      ‘1’ depressed on the rocker switch meant “on”, John.

    9. 9 Gilbert Renaut

      There’s a place in England that’s called “Hill Hill Hill” in three different languages, but I can’t remember what it is. Thanks for this, I am setting up a new desktop, and one speaker said don’t plug this in with the power switch on, and I didn’t know which was on.

    10. 10 Steven

      does anyone know if this symbol can be trademarked in the US?

    11. 11 Edward King

      To all, the symbol (-O) has nothing to do with 1 and zeros.
      It means OPEN CIRCUIT CLOSED CIRCUIT from the old knife switch schematic. When it was written 1 and zeros were meaningless. I once asked a PHD what he thought it meant and he said ” On ” ” Off ” well duh, true meaning=OPEN CIRCUIT CLOSED CIRCUIT!

    12. 12 Earl

      The only way I can ever remember which is the on and which is the off, is that the line (I never heard it was a ‘1’)is a straight line, as the power flows throught a power cord. The ‘0’ is like zero or no power flow through a power cord. Basically, I guess this is what Edward King’s comment was saying. What ever works for you.

    13. 13 jamida

      Great post, I didn’t realize such combo on-on/off switches were being made. Hopefully it will stop soon.

      On the other hand, why is the symbol for a closed circuit a line-segment and the symbol for an open circuit a closed loop? This seems to be the opposite of what it should really be.

    14. 14 Esther

      When I worked at the science museum the guys in the workshop used to put selfmade stickers with “On” and “???” on all kinds of apparatus they made. Of course they didn’t mean to implicate a chicken would come out of the device, just a little bilingual word play you might enjoy.

    15. 15 Nathan Zeldes

      Hey Esther, looks like WordPress doesn’t take kindly to bilingual word play… it turned your Hebrew chicken into “???”! But I can deduce what you meant… :-)

    16. 16 Kendra

      I was thinking maybe you press it to turn on or to go into standby because Wikipedia said that used to be the old standby symbol before the crescent moon.

    17. 17 eric

      I think it’s nuts that anyone would ever use glyphs that are not obvious
      on something as important as a power switch. The first photograph above
      that says “on” and “off” is clear to all but the retarded (or non English
      speakers), the circle and line are not, even just based on a quick survey of
      people in the office. If you want a good language-independent symbol, how
      about a light bulb, and light bulb with lines coming out of it. Most people
      would say it’s obvious the one with the lines means light (power) on.

      It’s like the bit from Demolotion Man where the guy askes what the three shells
      are for in the bathroom. If you have to ask, that’s telling you it’s confusing.

    18. 18 Edward Yang

      I ran across your site as I took delivery of a new power supply unit for my PC from Thermaltake. Looking at the power symbol, it shows only the 1 and 0. Until I saw your site, I had no idea that it represents binary 1 and 0.

      Amazing that something as simple as a on/off illustration can cause so much confusion.

      Now I know that the 1 means on and 0 means off (recalling my old electrical circuits enginnering class from mechanical engineering undergrad). Pretty sad that I didn’t even know how to read the 1 and 0.

      Very helpful as I undergo out of PC testing of my new power supply unit so I don’t shock myself. Thank you!

    19. 19 Clueless in Seattle

      For years I’ve been puzzling over those two symbols, the circle and the line, that often appear alongside rocker switches.

      My first guess was that the circle must represent a closed circuit, i.e. “on,” and that the line, since it is in effect broken at each end, must represent an open circuit, i.e., “off.”

      But from my experience with the way they actually work, it seems that the opposite may be the case.

      Will in Seattle
      a.k.a. “Clueless”

    20. 20 Clueless in Seattle

      Hi again!

      I just found this interesting document that shows a bunch of symbols and says the circle with the line in it means “standby.”

      http://eetd.lbl.gov/Controls/overview/symbols1.pdf

      It also confirms that my surmise that the line and circle represented circuit diagrams was, indeed, totally mistaken.

      Oh well, live and learn, eh?

      Will in Seattle
      a.k.a. “Clueless”

    21. 21 Craig

      In the 3/9/10 issue of the NY Daily. News there is picture of the power button logo with the title “Power up! City flips switch on new official condom.” Now the logo will also be synonymous with a male penetrating a female or another male. Gives another meaning to the phrase “Power Up.”

    22. 22 greg

      I have been in the same quandary for years. The symbol is ambiguous. First off, it certainly seems to show a circle and a bar. No sane person would interpret the symbols as a zero and a one (though, apparantly, that is what they are). Intuitively, I see a circle which looks most nearly looks like a tube or open conduit. And I see a bar which most nearly looks like a bar (duh!), a block or a wall. So with no outside help this symbol will mostly naturally be mistaken, for I see the circle as an open conduit for electricity (meaning-“on”) and I see the bar as a block to the possible flow of electricity (meaning-“off”). If the symbols were used in any non-electrical on/off applications my assumptions would need to be rethought, but I have never seen these symbols used in mechanical applications. I have more than a basic background in computers and can do binary mathematics, so the zero/one idea works really well for me. But, why on earth didn’t they make it look unmistakenly like a zero and a one? Go figure.

    23. 23 Edward King

      Please See comment dated……..above
      Sep 2nd, 2008 at 4:55 pm

      To all, the symbol (-O) has nothing to do with 1 and zeros.
      It means OPEN CIRCUIT CLOSED CIRCUIT from the old knife switch schematic. When it was written 1 and zeros were meaningless. I once asked a PHD what he thought it meant and he said ” On ” ” Off ” well duh, true meaning=OPEN CIRCUIT CLOSED CIRCUIT!

    24. 24 Nathan Zeldes

      Edward, your view has been noted before, and is an interesting derivation… I remember those knife switches in old gear. But other views exist too, and all are welcome to state their opinions on this blog!

    25. 25 Edward King

      Mr. Nathan Zeldes,

      Sir, You are absolutely right, I apologize. My intent was not to mame, make fun of, I am above you or any other rude actions. As a humble electronic instructor, my passion gets away from me at times, I am so sorry. This site is the only one that has a practical understanding of the importance of this symbol. At the top there are 2 perfectly scaled pictures of both the text version and the glyph version of the symbol. Who ever picked these fine examples also have them aligned so the text “on” is above the “-” for clarity, brilliant. I use these pictures when I teach basic electronics to my students. As luck would have it, even if the symbol is misinterpreted as a digital 1 and 0 the on/off concept still works, a testament to how perfect the symbol is even in this modern digital age. Earl summarizes this elegantly also after my first post ” What ever works for you. ” Long live the on off symbol, it should be with us for a long time, I trust forever. Again, if I offended anyone I apologize in advance.

      Sir
      Thank you kindly

    26. 26 Nathan Zeldes

      No apology needed, Edward, though it is appreciated!

    27. 27 Kim Røen

      Wow, I always thought the symbol was a representation of the Tubular pin tumbler lock (http://en.wikipedia.org/wiki/Tubular_pin_tumbler_lock)

      I don’t know where I got this from, I just associated the act of locking and unlocking (making the item unavailable and unavailable) with turning something off and on.

      The symbol looks like it I think, take a look at this picture:
      http://www.royallock.com/cam/images/lock1f.gif

      Quite interesting I think, but my dad used to have a computer which was turned off and on with a lock and key like this one.

    28. 28 Nathan Zeldes

      @Kim Yes, I remember those round locks… they came standard on all early IBM PC machines. Not that I ever saw anyone use the lock, and eventually you could lock the machine in software so they disappeared.

    29. 29 Jessica

      I just think it’s funny how many people get confused about it… when it seems like even if you forget which is which… it’s obvious. Look at which is set, and look at the device. Is the device on? Then that’s the on symbol, and the other means off.

      :-P

    30. 30 paul

      broken circle with a line = on/standby – power supply is not disconnected from the device.
      circle with a line completely inside = on/off – power supply is fully disconnected.

    31. 31 Joe

      the bastard version to think of it is actually standby mode where the computer is ON but really its off in a power saving mode?

    32. 32 Alex the OGRE

      Glad Im not alone, even though Im not clueless, I am confused just north of Seattle!

      My button broke from the front panel, so I used a vial cap to end denting my finger on the tiny point there.

      Recently, something is locking up my system and the button worketh knot! So I have to unplug it instead, to do a restart.

      In the end, just goes to show that engineer types dont live in the real world and dont want to be bothered by the facts.

      Best proof is in the owners manual for my cell phone from Motorola. Half the icons on my screen are not detailed and the 800# doesnt know either.

      Bottom line, keep the power connection in reach at all times and send the enginners along with the lawyers to the same precipice as THELMA & LOUISE!!!

      HARRUMPH!

    33. 33 Matt

      I am under the impression that the 1 and 0 on the new rocker type switches came from the electronics guys, where 1 in computer bytes is on and 0 is off. It’s bianary language. Where that leaves the rest of us is guessing. welcome to the computer age!

    34. 34 jeff

      I am not positive the broken circle with the “one” inside is completely “off”, i.e. there may still be some voltage present. I believe the full circle indicates a fully “off” state. Similar to TVs and motion sensors, even when “off” (no picture or light is off) it is in a standby state and there is still voltage present, therefore a difference in the symbols.

    35. 35 jim

      Observe your keyboard and your switches, on the switches for your personal fan or your string trimmer gas powered. The marks are the capital letter O note it to be round, not oval, and the capital letter I. In 1820 (note the oval), a man by the name Ampere used the capital letter I for intensity of current a french phrase, to describe current flow in a closed circuit. The knife switch marking uses Ampere’s I and O. When would a gas string trimmer switch be used as a digital representation by multiple applications such as IIIOIIIIOIOOO? Never.

    36. 36 Jon doe

      actually, according to wikipedia:
      IEC 5007, the power on (line) symbol, appearing on a button or one end of a toggle switch indicates that the control places the equipment into a fully powered state. It comes from the binary system (1 or | means on)

      IEC 5008, the power off (circle) symbol on a button or toggle, indicates that using the control will disconnect power to the device. It comes from the power system (0 means off)

      IEC 5010, the power on-off symbol (line within a circle), is used on buttons that switch a device between on and fully off states.

      IEC 5009, the standby symbol (line partially within a broken circle), indicates a sleep mode or low power state. The switch does not fully disconnect the device from its power supply. This may appear on a toggle switch opposite a power on symbol, alone on a pushbutton that places the device into a standby state, or alone on a button that switches between on and standby.
      Alternatively, under IEEE 1621, this symbol simply means “power”.[2]

      A crescent moon, indicating sleep, is added by IEEE 1621 as a replacement for the standby symbol.

    37. 37 Kevin Luck

      I always thought that it originally came from “In service” (I) and “Out of service” (O). They then got morphed together to form the combined symbol when sequential switches became available rather than just toggle switches or separate On and Off switches.

    38. 38 Nathan Zeldes

      Kevin gets the “most original idea” award so far… but I greatly doubt this explanation. After all, by the same token they could stand for “Inoperative” and “Operative”…

    39. 39 JonBoy

      Really, the discussion should end. The answer has been revealed. John Doe has quoted the ultimate authority a couple of posts above this one. The symbols are derived from binary “1” and “0”, and each variation has a very specific meaning. And, if you are an old fogey like me, you will know, contrary to a previous post, that these symbols did not even exist on power switches before the computer age and binary logic.

    40. 40 Matt

      @JonBoy
      Wikipedia is hardly the “ultimate authority”, it’s just stuff written by people like the author of this post. Note that the Wikipedia page referenced[1] has [citation needed] for the I and O symbols where it says they come from the binary system. If you read the talk[2] page there it was added because there was no source cited and the person that wrote it just assumed or read it somewhere like the author of this post.

      @jim
      You explained that the I means current, which makes sense since that’s very commonly used in this sense (i.e. Ohm’s law). You never said what the circle stands for though, any idea?

      It’s so stupid that such a common and important symbol has no known meaning and the general population has no clue what the symbols mean or where they came from.

      My opinion is that switches should just have a little dot for the On position and nothing at all for Off.

      [1] http://en.wikipedia.org/wiki/Power_symbol
      [2] http://en.wikipedia.org/wiki/Talk:Power_symbol#stylised_1_and_0.3F

    41. 41 Matt

      Well I can’t seem to locate the official IEC 417 where these symbols are defined but I keep seeing this table on number Google results that clearly states they don’t mean one and zero:

      http://www.ee.uwa.edu.au/~allend/Symbols.html

    42. 42 hjzhang

      It is confusing to the extreme: now that I know what 0 and l mean. Before, the 0 was led to mean On and the line meant the line in the sand, do not cross (shut/blocked). Now we know it is entirely opposit: O=off instead of on and l means the wire the electricity goes through, it is not so for the symbols on Diode or rectifier where the up and down straight line is ‘blocked’. At the lease, it is confusing and not uniform.

    43. 43 Shawn Clarady

      It was found in a crashed UFO and yes it is a 1 and a 0. Binary code is universal.

    44. 44 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).

    45. 45 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.

    46. 46 Philip
    47. 47 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).

    48. 48 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?

    49. 49 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.

    50. 50 Nathan Zeldes

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

    51. 51 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?

    52. 52 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).

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