It doesn't seem to me that a railway semaphore describes a semaphore better than any other generic signalling object, like a flag or a stoplight. Even something like a keyring (people take keys off the ring to access locked resources, then return them when they're finished) is more descriptive. So why is a semaphore called a semaphore?

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    The real question is why the two operations on a semaphore are called P and V.
    – user251748
    Sep 25, 2017 at 14:05
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    I searched for semaphore etymology and the only links I found were for why the operations are called P and V. Nobody explained why they were called semaphores!
    – eyqs
    Sep 25, 2017 at 16:27
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    What is the origin of the word etymology? The real question is: why is it called a 'flashlight'? ('torch' is right out.) Due to ancient battery technology where the voltage would fall very fast as current was drawn, you had to blip the flashlight on for just a moment at a time. The good old days, eh?
    – user251748
    Sep 25, 2017 at 16:33
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    IIRC P and V are initials the first letters of the words for "set" and "get" in Dutch, or something like that.
    – user253751
    Sep 27, 2017 at 3:42
  • Original semaphore: en.wikipedia.org/wiki/Semaphore_line
    – nvoigt
    Apr 23, 2018 at 8:43

1 Answer 1


It's the metaphor that the creator of the idea proposed.

The idea was proposed by Dutch computer scientist Edsger Dijkstra in the early 1960s.

I don't speak Dutch, but looking at the paper he wrote on the subject, it looks like he talks about seinpaal which translates in English to "signal post" or semaphore.

He describes the concept as:

Go to sleep, we'll wake you up again when you need to continue.

This sounds to me like a metaphor for a train driver approaching a semaphore and stopping until being allowed to resume.

semaphore and train

He says in the paper:

From now on I will call the logical variable that prohibits the machine from continuing on as seinpaal (or semaphore)

If created by an English-speaker, they might have chosen something else. But for a Dutch-speaker in the 1960s, this analogy must have made the most sense to describe what the idea was.

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    I was going to write an answer about en.wikipedia.org/wiki/Token_(railway_signalling), but from your description of "go to sleep, we'll wake you when you need to continue", it sounds more like a railway signal.
    – Gaurav
    Sep 26, 2017 at 17:13
  • @Gaurav yeah, that Token thing is either brilliant or raving mad. I was stunned when I saw a TV show explaining it. But, we all carry phones nowadays that allow us to proceed with things, so...
    – user251748
    Mar 23, 2018 at 0:14

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