The notes on Leonardo da Vinci's famous Vitruvian Man image are in mirror writing.

Mirror writing is formed by writing in the reverse direction than is natural for a given language such that the result appears normal when it is reflected in a mirror. It is sometimes used as an extremely primitive form of cipher.

One can create mirror writing without practice by writing down some text with a thick pen on a thin sheet of paper, flip it over and then trace the mirrored text. Use an additional sheet of paper if you do not want the normal text on the back side to show.

Leonardo Da Vinci

Leonardo Da Vinci is famous for having written most of his personal notes in mirror and using only standard writing if he intended his texts to be read by others. There are two popular theories on why he did this. Leonardo Da Vinci was left-handed causing the ink to smudge easily if he wrote in standard writing. He may also have wanted to protect his ideas from being stolen by others or hide them from the Roman Catholic Church with whom his scientific findings sometimes collided.