Leonardo’s notebook writing famously runs from right to left across the page, with its letters reversed, best read with the help of a mirror.

Scholars have debated why Leonardo wrote this way. One theory is that he wanted to protect his ideas from the prying eyes of those who might copy them. But that’s unlikely, since he was planning to publish his ideas in a book. Another is that he was protecting himself, that his heretical ideas could be used against him if they could be made known. Also unlikely, as he wasn’t much more heretical than many of his time. But the most plausible reason is that he was left-handed, and pulling the pen across the page from right to left wouldn’t smudge the ink. I know that doesn’t explain the reversed letters. Chalk that up to his bizarre genius.

Experts who evaluate the authenticity of unsigned Renaissance drawings that are attributed to Leonardo first check the direction of the shading. If it doesn’t slant downward from left-to-right, chances are it’s not a Leonardo. I wondered about this downward left-to-right slant when I read it in Robert Wallace’s The World of Leonardo (page 31). Why would a leftie push the pen from left down to right, a sure way to smudge the ink? But then I realized he must have pulled the stroke upward, moving the pen diagonally from right up toward the left. When you’re shading, of course it makes sense to start at the object in the foreground and draw your shade lines away from the subject. Can you see how they slant upwards toward the left in these beautiful sketches? I could stare at Leda’s face all day.

Detail from Vitruvian man notebook page, 1490

Study of Arms and Hands, 1474, a sketch by da Vinci popularly considered to be a preliminary study for the painting “Lady with an Ermine“

”’Study for the Head of Leda”’ (c. 1505 – 1507)