They're so powerful, ubiquitous and unnoticed, that it's useful to see what happens when one doesn't work.
In the late 1970s, was lucky enough to meet the HP programmable calculator, the 9100A. It was the size of a microwave oven and you could store instructions on little magnetic strips to execute as programs later.
It didn't have an alphanumeric keyboard, the commands available were on the keys - as with later HP handheld calculators, one key could have more than one meaning. The screen showed the x,y and z registers. You could connect a plotter to it, and the physics department had given me the exercise to plot the orbit of the earth around the sun.
Mostly everything made sense, but there was one part of the set-up that puzzled me considerably.
There were a few keys connected to 'flags'. You could 'set' or 'clear' a flag, and then test to see if a flag was 'set' or 'cleared'. I could understand why these were useful, because they allowed simple binary decisions to be made, based on earlier results.
What didn't make sense to me was the term 'flag', and it being used in this way. I knew about flags as things made of cloth that were run up flagpoles by people who were keen on that sort of thing - the same sort of people, usually, who were keen on uniforms, marching about, and killing people.
It wasn't clear to me how these flags were connected to the ones on the 9100. What was, to me, particularly puzzling was how anybody would think of a flag as 'set' or 'clear' and how you'd know which state a flag was in. If the flag metaphor was used, I'd have expected 'raised' or 'lowered' or 'half-mast' to be the terms used.
I wasn't able to resolve this from the manual, because it seemed to assume familiarity with flags used in this way, and nowhere described where the metaphor came from.
It was about four years later, when I visited California for the first time, that I discovered what it was all about. I'd seen photographs of letterboxes in America, and noticed them in films. They were cylindrical, with a flattened bottom and a hinged door. I was surprised by their uniformity, being used to seeing many different designs, not just one.
The other feature they had was a key-shaped bit of metal mounted about half way down. This, I discovered, was the source of the metaphor. This wasn't just a bit of decoration, but a signalling device. People who lived in the US knew this, and called the bit of metal a 'flag'. The protocol was that, when he deliver the post, the postman would raise the 'flag', thus 'setting' it. This signalled to somebody looking across the front garden that there was new post. When you collected the post, you'd put the 'flag' down (clearing the flag), to indicate that the postbox was empty. [I've now been corrected, you can see in the comments below. Apparently you, the owner of the letterbox 'set' the flag to indicate a letter was waiting to be collected, and the postman 'clears' it, by putting it back down, indicating that the letter has been collected, and that you have new post - the opposite of how I'd understood it... making it even clearer how difficult it can be to understand a metaphor from an alien source].
The alternative etymology, that might be behind the letterbox usage, is the 'flag' used on a taximeter to indicate if it is free, or 'for hire', or occupied, 'hired'. The OED doesn't give etymology for the computing use, so this isn't certain.
Clearly, the makers of the HP9100A, who lived in California, and worked for Hewlett-Packard, who'd paid for me to travel to California, as their employee, knew all about this, and thought the principle universal. Which was why there was no explanation of what 'setting' or 'clearing' these 'flags' was about.
In the years since, when meeting difficult metaphors, or thinking of a metaphor to describe something myself, I've often remembered this problem. If you are going to use a metaphor, you have to make sure that your audience will be familiar with whatever it is that you are going to use as a metaphor. Otherwise it will fail to communicate your idea - or, if it does, it'll involve a special effort of interpretation by the reader to work out what you were getting at.
