This Week on Mystery Theatre:Cracking the Code

The irregular tapping came from the other side of the sheet-metal wall that separated Paul’s and Harold’s cell from the totally enclosed tank for desperados next door.

Experimentally, Paul tapped on his side.

“Twenty-three—eight-fifteen,” came the reply. Paul recognized the schoolboy’s code: one for A, two for B … twenty-three—eight-fifteen” was “Who?”

That’s a rudimentary version of tap code (also known as knock code) depicted in Kurt Vonnegut’s Player Piano, a novel that a recent report on NPR...