Well, there was writing in invisible ink (e.g. lemon juice) in the whitespace of apparently normal letters. Or having the first word of each sentence spell your

Well, there was writing in invisible ink (e.g. lemon juice) in the whitespace of apparently normal letters. Or having the first word of each sentence spell your secret message. Etc. Which reminds me, when at school we had code-names for various drugs so they could be freely spoken of within earshot of anyone, including parents/teachers. Claude = cigarettes, Harry = alcohol, George = marijuana etc hehe. Worked very effectively.