Yesterday's answer was carbon paper, and I was all prepared to give credit to those who said the not quite technically correct answer of carbon copy, but then Konrad swooped in with a late-night dead-on answer and stole trivia glory from the CC crowd. Congratulations, Konrad!
On a related note, I have no idea who invented blind carbon copy paper. Here's today's question:
How many dots make up a single Braille cell?