enter image description here

What was the thought process behind whoever designed the I/O on/off button?

To me, it seems extremely confusing whether or not I is on, or O is on, and I still get pretty confused today as to which is which.

Why was this designed this way? From a user-experience standpoint, it doesn't make much sense, wouldn't ON/OFF work better? Better yet, why is this still considered the norm in today's society, and not ON/OFF?

Is there anything else better than I/O that is better for the user? Or am I best sticking to I/O?

  • Do you have a situation where you're looking to use a power button such as this? What situation is that as we can cater the solution to the actual problem. If it's just general curiosity then this isn't the best place for such a question - we need answerable questions that actually provide a solution to a specific problem.
    – JonW
    Commented Aug 5, 2013 at 7:54
  • As the image shows - what matters is that it lights up to show that it's on.
    – PhillipW
    Commented Aug 5, 2013 at 15:24
  • @JonW Edited to fit your guidelines
    – yuritsuki
    Commented Aug 5, 2013 at 16:25
  • @PhillipW Note, I have quite a few switches at home that have I/O, but don't have a nice light behind to tell me if it's on/off. I just pulled a stock image because I couldn't find anything else that looked more clear
    – yuritsuki
    Commented Aug 5, 2013 at 16:28
  • 1
    I've just had to dig this question out as I have a new fridge with a (non illuminating) switch marked 1/O and still couldn't remember which setting is supposed to be 'on'...
    – PhillipW
    Commented Oct 27, 2013 at 20:06

7 Answers 7


This has to do with binary numeral systeem. 1 for on, 0 for off. This way it's understandable for everyone around the world, since not everyone understands English (ON/OFF).

It's also readable from 2 sides, where ON/OFF is harder to read.

enter image description here

  • 2
    +1 for pointing out that it's readable upside-down as well!
    – 17 of 26
    Commented Aug 5, 2013 at 17:41
  • 10
    O and I are the esperanto of switches: they're made for everyone, but only experts understand them.
    – bigstones
    Commented Aug 6, 2013 at 11:03
  • 3
    I think there a way more people who understand ON/OFF rather than the binary numeral system.
    – hansmaad
    Commented Mar 19, 2015 at 13:31
  • 1
    I'm a nerd and always get confused by the traditional 1/0 labels. I interpret them as a straight line (blocked, off) and an open circle (opening, flow, on).
    – Kip
    Commented Mar 19, 2015 at 23:48
  • 1
    I am a computer nerd with a full understanding of binary. I also understand circuits. I never never remember if its an open (I) and closed (O) circuit or if it's a binary zero/one. Since it's almost always related to circuitry, my mind kind of goes that direction. I can see why people would find it confusing Commented Jun 4, 2019 at 23:21

Shamelessly taken from Wikipedia:

English words were replaced by the universal numeral symbols 1 and 0 to bypass any possible language barriers

I - 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).

O - 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 binary system (0 means off).

  • 2
    Which rather misses the point that as most people don't think in binary - that the symbols are useless.
    – PhillipW
    Commented Aug 5, 2013 at 15:26
  • @PhillipW Most people don't think in English, either, so I'd say the change wasn't really a step backward for the international audience. Maybe not a giant step forward or anything, but for switches where a light or other "obvious" indication isn't possible, you have to use something, and simple symbols are better than words taken from any given language.
    – Geobits
    Commented Oct 27, 2014 at 22:23

It's from physics, I guess.

"I" symbol means the current goes through the system (imagine the 'I' being a line, like a circuit connecting [power to the device])

"O" symbol means the current does not go through the system. (the circle is an open circuit, having no power flowing through it)

  • 4
    Strange reasoning to me. I think just the opposite when looking at the symbols: O ought to mean current going around in a circuit, I is a barrier stopping it.
    – jamesqf
    Commented Feb 23, 2015 at 19:26
  • But a circle is circuitous. A line is not a circuit. Ugh.
    – Alan H.
    Commented Sep 23, 2018 at 1:47

Interesting question. Some things we consider obviously Yes and No are different in different cultures.

Consider the sony playstation controller: O in japan means "good" or "approve" and X means "bad" or "cancel"

For the US they had to change the controller buttons for X to mean approve and O to mean cancel. Fascinating, right?

x means cancel and o means approve in japan.

Checkmarks pretty much always mean "yes" or "approve" so whatever your choice for the approve side of things is, it should be unambiguous.

Will think about potential alternatives...


Color is also an effective way of portraying on/off. While there may be some cultural differences when it comes to the meaning of certain colors, it's definitely a familiar use case to represent on with green and off with red. Similar to how a traffic light would function. In UI, a switch like in the picture below is widely accepted and used in applications:

enter image description here

This variation can be enhanced even further with the inclusion of symbols to address usability concerns. While the example below still uses I/O for international users, the on/off state becomes a bit more clear with the inclusion of the colors and slider element:

enter image description here

  • What if the user is colorblind who can't tell the difference between red and green? Traffic lights work because of the location of the red and green. Commented May 5, 2020 at 5:48
  • @locationunknown Yes, that's definitely a usability concern. To address that, you would need some sort of symbol that acts as a backup. For a switch like this though, I don't think I/O would make sense as only one symbol is displayed at a time.
    – mbrailov
    Commented May 5, 2020 at 6:12

In some cases whether lights come on or not is not helpful. Some car battery chargers have lights which come on if the battery is partly charged, and the light indicators hardly change depending on the switch position. It is therefore essential to "think" in binary, which as noted, not everyone does. Hence some people will keep coming back to this or similar sites to check.


O is for Open Eye.

I is for Squint because working.

Not the answer you're looking for? Browse other questions tagged or ask your own question.