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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 Aug 5 '13 at 7:54
  • As the image shows - what matters is that it lights up to show that it's on. – PhillipW Aug 5 '13 at 15:24
  • @JonW Edited to fit your guidelines – yuritsuki Aug 5 '13 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 Aug 5 '13 at 16:28
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    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 Oct 27 '13 at 20:06

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.

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  • +1 and I would like to add that 1 can also mean "yes" and 0 accordingly "no" interpreted in binary code – tillinberlin Aug 5 '13 at 14:47
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    +1 for pointing out that it's readable upside-down as well! – 17 of 26 Aug 5 '13 at 17:41
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    O and I are the esperanto of switches: they're made for everyone, but only experts understand them. – bigstones Aug 6 '13 at 11:03
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    I think there a way more people who understand ON/OFF rather than the binary numeral system. – hansmaad Mar 19 '15 at 13:31
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    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 Mar 19 '15 at 23:48

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).

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    Which rather misses the point that as most people don't think in binary - that the symbols are useless. – PhillipW Aug 5 '13 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 Oct 27 '14 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)

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    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 Feb 23 '15 at 19:26
  • But a circle is circuitous. A line is not a circuit. Ugh. – Alan H. Sep 23 '18 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...


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.

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