A writing direction that suited the hand
For a left-handed writer, ordinary left-to-right writing can put the hand over the words just made. Reversing the direction changes that geometry: the hand moves after the pen rather than sweeping across the fresh line. That makes mirror writing a practical adaptation, not merely an unusual code.
Leonardo’s notes were written with reversed letter forms as well as in the opposite direction. To read them in the usual way, a reader can hold a page to a mirror, which flips the script back. The effect looks deliberate and puzzling now, but it follows from a simple change in the direction of the pen.
Private notes, not necessarily a secret cipher
Leonardo filled notebooks with observations, sketches, and working thoughts. Because the pages were for his own use, he had little reason to make every line easy for another person to read. Some historians have suggested that mirror writing helped conceal the contents, but the handwriting alone cannot establish that motive. Its practical advantage for a left-handed writer offers a straightforward explanation without requiring a secret message.
The memorable twist is that a page that appears to run backward may reveal less about what Leonardo wanted to hide than about how he wanted his writing hand to move.
