Visual Signs and Symbols
When I was travelling last week I spotted this sign on the side of the plane engine:
This is not a men's room? No urinating on the engine please?
What is a men's room sign doing on the side of a jet engine? To me it looks like an example of how symbols evolve over time and take on a different meaning. The symbol used to be a generic "male" symbol. In the, um, waste management context, it was used as a secondary symbol, under a "Toilet" sign to separate the men and women. (If you are American and the topic of toilets troubles you, I'm sorry :-))
The male and female symbol has been promoted to take over the meaning of Toilet. It is so widely used for this that it takes on the meaning of Toilet even out of context. Is this a loss or a gain? I believe it is probably more of a loss than a gain.
There are a number of other examples of symbols taking on new meaning. The peace sign ☮ used to mean peace, it now means "the 60s".
There are also numerous examples of symbols that depicts objects that no longer exists. This is generally the problem with symbols. They don't age well. For example the floppy disk used for "Save" (look it up in Wikipedia, kids). There is a generation growing up who has never seen a floppy disk, yet they a still merrily clicking it to save their torrent files. The floppy disk has gone from being a symbol (a metaphor) to being a sign.
Signs and Symbols are not the same
There is a difference between a Sign and a Symbol, in the semiotic sense. *
A (Visual) Symbol stands for something. It is connected to something through some form of relation (visual similarity, history, etc). It often depicts a real world object and is often metaphorical.

A (Visual) Sign on the other hand signifies something. Its form is arbitrary and it simply has to be learned. For example currency.
37signals has recently added currency signs to some of their their software. The sign to the far right is supposedly a generic currency symbol. Even if I have if right here on my home keyboard, look: "¤", I never knew what it was until I read it on their blog today. (The sign is not on my UK work laptop keyboard.) The sign was introduced in 1972. I guess it is a good example as any that you don't learn signs that you don't need in your daily life. It takes a lot of time and effort to establish a sign.
Symbols as metaphors
The advantage of symbols used as metaphors is that they can symbolize "invisible stuff" without requiring learning. You can understand their meaning without explanation or prior experience.(And hopefully it's the same meaning as the designer intended). Many designers believe metaphors are superior for this reason. You don't have to know or learn anything, and that must be the ultimate ease-of-use, right?
Well, it would be if there were an easy to understand metaphor for everything. Unfortunately there isn't. Symbols are limited, they can have a hard time representing abstract notions or verbs. What is the metaphor for "Power", "Stop", "Yes", "No", "Fun", etc? There are a lot more abstract notions and verbs than real world objects that can be given a recognisable shape on an icon.
Signs on the other hand are practically limitless. The number of individually distinguishable shapes are, well, more than plenty. The problem with signs is that their meaning has to be agreed upon beforehand. If you haven't seen a particular sign before, it's hard to know what it means. And to be really useful to designers, signs has to be understood in the same way globally.
Cultural differences
In reality, not many signs are globally understood. What signs are? My guess would be the numbers from 0 to 9, the exclamation mark, the plus sign, Play/Pause/Stop, the dollar sign... maybe a dozen more?
Someone should set up a website to "crowdsource" how various signs and symbols are interpreted around the world. Maybe that already exists(please share in the comments if you know about such a site).
MotoFone
The Motorola design team tried to tackle universal icons a couple of years back with the MotoFone. The phone is an ultra-low-cost phone designed for emerging markets. It uses icons and voice prompts instead of text for the menu system and the design team tried to come up with a set of "universal icons" (shown below) for basic phone functions. Can you understand these without explanation?
Most should be fairly understandable; Trashcan is delete, Envelope is message, Book with plus sign is "add to address book". These metaphors are close enough. The 123, ABC and OK are straightforward direct depictions and not metaphorical.
"No reception" is where the designers ran out of metaphors. The House with a cross means Out of Network. The other icons feels user-centric; it's my contacts, my messages. I own them. The house with a cross over is not my home. It's the the phone operators home so to speak and I don't own it (If you're willing to stretch it, you could say that I rent a place there). It is clearly system-centric. How do you draw a self-explanatory symbol for "no reception"? I'm not sure it's possible.
MotoFone icons with explanation
Ovi
Another attempt at universal icons, this time with a Web 2.0 slant. These are "Social Networking" icons from the Nokia Ovi website (and will be used in upcoming Nokia phones as well).
Nokia is currently changing the icons and compare the old stylish charcoal gray (standard designer issue) to the new colour version below. The new version is both more detailed and more playful. The game icon has changed from Tic-Tac-Toe to a game controller. A Nintendo game controller has a stronger identity than Tic-Tac-Toe on a global basis? Well, there should not be any reason to doubt that, as with the Motorola icons above, these has been thoroughly researched according to the respective companies.
Nokia Conversations will post a video interview next week with one of the head designers in the Nokia Design team who were responsible for these icons. Might be worth a look.
On and off
You might think that a symbol for a basic function like on/off should be possible to agree on, but no. The US version of the iPhone uses the text ON and OFF on switches, the international version(s) uses the I/O sign. Is the US is less visually literate than the rest of the world?
Btw, a pet peeve of mine is misuse of the IEC power symbol. Look around you right now, and you will probably see it several places. Your mobile phone, your monitor, your PC. The symbol is 0 (zero) for "off", 1 (one) for "on" and a a combination of the two for "standby".
IEC power symbols are universally recognized. Except in the US.
Most mobile phones (Nokia, Sony Ericsson, Samsung, LG and Blackberry) uses the "power on/off" symbol to turn the phone on or off. Motorola used to use either, but seem to have settled for "on/off" on recent phones. HTC and Palm uses the "standby" symbol on the on/off button. Software like Yahoo Go also erroneously uses the Standby symbol to exit the application.
The meaning is in people, not in the symbol
This means interpretation of symbols may and will vary.
Symbols change their meaning according to context. A Crescent moon can mean sleep when printed on a button, but mean other things when printed on a flag, or in a book about astronomy, etc.
The difference between signs and symbols can be unclear. Signs may contain symbolic properties and vice versa. A dollar sign like "$" is a simple sign, but when someone writes Micro$oft, it gets a ton of symbolic meaning. Sign and symbols are a central to a rich visual vocabulary and is really at the core of what visual design is all about.
Care for the little ones
The few globally understood symbols we have at our disposal are precious gems. The main reason signs (and symbols) change their meaning over time is that they are introduced into a different context with a slightly different meaning, and the new context may become more prevalent than the old one.
To be truly "intuitive", the meaning of a symbol have to be transparent to someone who have never seen it before, and that happens less often than you may think! Watering down the few globally understood signs, symbols and metaphors that we have might not be in our own best interest as designers. For example, many designers don't like the power symbol, it's ugly, it's designed by engineers, it doesn't feel, um, powerful enough. Let's invent something else! It's like saying "I don't like the form of the letter "A", it doesn't really say 'aaaaah' to me".
Forget it, spend time on stuff that really needs improvement and let's instead try to strengthen the few globally understood symbols we have by using them consistently and correctly.
*) I did not link to the Wikipedia entries for Signs, Symbols and Semiotics in the text because the Wikipedia articles are somewhat contradictory. Semiotics belongs to the soft sciences, where nobody is ever able to agree on anything. (They would be out of a job if they did.) Anyway, my point is the same regardless of nomenclature: Some signs and symbols are worth saving because they are globally understood.

Some comparative evolution tables of icons at http://www.guidebookgallery.org/icons
(via the resources in Rui's http://the.taoofmac.com/space/Icons)
br -d
Posted by: David Mery | September 18, 2008 at 12:00
I don't know about you, but I understood what that meant straight away. Simply put, "No People". In the context of the jet engine, I assume it means don't come near here.
Where I live I see these rounded stick people almost everywhere in contexts nothing to do with toilets. For example in the Subway cars in Seoul a similar stickman with a cane is hung over certain seats to designate, "reserved for seniors". Or when driving a stick boy with a ball represents a play area or a child with a backpack holding their mothers hand represents a school zone. I've seen that exact same man as a light for the "Don't walk" symbol at traffic intersections. I think these rounded stick figures are the universal symbol for people. When hung in front of doorways the context suggests a bathroom, but on a jet engine I think it's pretty clear that it has nothing to do with toilets.
Posted by: Ian White | September 17, 2008 at 03:21
Same applies for words. for example, labels such as 'menu' on familiar music devices look descriptive but actually signify behavior that must be learned. So for icons and labels, ease of learning is more important than recognition.
Posted by: Maurice | September 15, 2008 at 10:40
It takes a lifetime for a symbol to become a sign (as you mentioned about the 'save' icon). A few more good examples are:
1. Envelope = Mail (Even though people hardly write mails and post them through letter boxes).
2. Dustbin = Delete/trash (The dustbin is a classic western dustbin, but the symbol is now recognised worldwide).
3. Alarm Bell (Not sure how many people still remember that this symbol comes from the big 'gong's' on the bell towers (http://en.wikipedia.org/wiki/Bell_tower)
These are some of the physical objects. Then there are references picked up from the street signage.
Use of Yellow Triangles, Hexagonal Red boxes etc...
I am still curious about the origin of a lot of symbols and maybe should start a blog about it :)
Posted by: Sajid | September 15, 2008 at 09:46
Re: "What is a men's room sign doing on the side of a jet engine?"
I've been intrigued by several similar notices I spotted in Seoul. They're currently the latest ones at http://www.flickr.com/photos/acrostich/tags/signs
@Johannes Baeck
This research project in how to create a signalisation that can work over long period of time was indeed fascinating. It can be found at http://downlode.org/Etext/WIPP/
br -d
Posted by: David Mery | September 13, 2008 at 05:08
The symbol I've come to associate with "no reception" is the radio mast with a cross mark near the top.
This meaning has been made particularly clear because several mobile OS interfaces use variations of the symbol to represent different states of radio reception. The radio mast symbol has a consistent position on screen for depicting signal strength with varying numbers of bars coming out of the mast, so the meaning is obvious when there are no bars on the mast and a cross instead.
As for the IEC power switch symbols, I think the standby switch is abused quite a lot because it's the most distinctive. It's a victim of its own good looks.
Posted by: Ben Bodien | September 10, 2008 at 11:44
Great Article!
The Floppy Disk as a symbol for saving is a really good example for a symbol dieing out. I'm curious what will follow, or will it still be the "save" symbol in ten years?
I recently read an article about the difficult question which symbol to use for atomic waste. As this symbol will be important for a really long time it is nearly impossible to find a symbol which will be understood in about 24000 years!
Posted by: Johannes Baeck | September 10, 2008 at 09:22
Entirely aside from the context, there are micro-cultures all over. The set of people who should be walking around anywhere near an operating turbofan is a very small group, who are supposed to have prior knowledge.
If you care, this is saying "no people" forward of the red line. Which lines up with a diagram explaining the "danger areas" of the engine. You can get sucked into it or blasted by exhaust in the red zones on the diagram.
The diagram works for this population because they already know to expect this danger, and just need to be reminded of it, and how the danger area of this engine varies from others.
Posted by: Steven Hoober | September 10, 2008 at 05:27
My company is a usability consulting company in India. We have run in-depth comparative usability studies using motofone in India. The motofone f3 was the only phone people actually threw on the floor with frustration.
Most participants could not understand a single icon or even make a call from the address book! So much so for "intuitive" icons and "low cost".
Posted by: Ripul Kumar | September 10, 2008 at 05:17