I saw a TV interview with a Vietnam War American POW who said they would tap signals on the wall of cells, and to tell if the person tapping back was a prisoner

I saw a TV interview with a Vietnam War American POW who said they would tap signals on the wall of cells, and to tell if the person tapping back was a prisoner or a guard, they would tap Shave-and-a-Haircut (see http://en.wikipedia.org/wiki/Shave_and_a_Haircut http://en.wikipedia.org/wiki/Shave_and_a_Haircut ) because the guards just could not get it right. The WP page gives a link to this story.