Famous people
Leonardo da Vinci wrote most of his notes backwards, in mirror writing.
His thousands of pages of notebooks run from right to left with reversed letters, readable in a mirror. He wrote normally when the text was intended for other people.
Why is this true?
Leonardo was left-handed, and writing right to left kept his hand from smudging the wet ink. Some historians also think he liked the privacy, although it was never a real code.
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More about famous people
Famous people accumulate stories, and only some of them are true. The entries in this category stick to documented episodes: the patents, the letters, the odd habits recorded by people who were actually there. Where a story is commonly retold in an exaggerated form, the explanation says so, which makes these facts safer to repeat than the version that usually circulates online.