After a vacation in, then, Czechoslovakia I received a letter from the police. The letter was artfully folded so that the text would overlap at 90 degrees (if v

After a vacation in, then, Czechoslovakia I received a letter from the police. The letter was artfully folded so that the text would overlap at 90 degrees (if viewed with a light source behind the folded letter). I always reckoned that was done to scramble the letter from unintended reading. Normally the C3 or C4 envelopes used by government would be printed on the inside with a mass of letters, but in a way that matched with how the letter paper inside would be folded.