Fast but not terribly secure. That’s the problem with flag semaphore, which has been used as a simple communication system since about 1800. The idea is this: Someone stands on the deck of a ship with two flags, one in each hand. He then does a dance a little like a cheerleader, wagging the flags to specific locations to signify different letters. Here is a sailor spelling out “SMILE:”
It works, and it’s still taught and used today in case of communications failures. Semaphores can be seen for miles if you have a good telescope, they work in all sorts of weather and they aren’t dependent on electricity. Flags don’t need recharging.
The problem with semaphore is that there’s no security to it. Unless you invent your own code, your enemies can see the flags and decode them as well as your friends can. It’s one thing to signal “HOLE IN BOAT SINKING FAST” when you are looking for help, but if the enemy fleet knows they’ve all but sunk you that’s another whole thing.
Semaphore pops up in weird places, because it’s been a big part of our culture. Today’s “peace sign” is actually a combination of the semaphores for “N” and “D” (meaning “nuclear disarmament.) See, and you thought it was a chicken foot this whole time.