When I was in college, my best friend from high school came up with a novel way to identify his print-out in the computer lab. It was a large room full of terminals, with a DECwriter dot matrix printer at one end. Students could print to the printer, but there was no good notification system that the print-out was done. You had to wander over after a while and hope that it was ready.

The dot matrix printer made a buzzing sound from the wires in the print head striking the paper. So my friend ended his files with patterns of increasingly tightly spaced vertical bars:

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When the file was printed, the typical buzz of the printer would be interrupted by a distinctive rising tone caused by printing the bars. It could be heard across the room, announcing that the file was ready.