If you were to take a look at any of Leonardo da Vinci’s written words that he himself put to the page, they would be inscrutable to the modern reader, principally because they’re in 15th-century Italian. As it turns out, however, at least some of his writings, according to Mental Floss, were difficult for his contemporaries to make out as well. Specifically, they were written in reverse and could only be read if held to a mirror — another factoid about da Vinci that suggests he was hiding something.

In fact, there was a much more practical reason da Vinci wrote in reverse: he wrote left-handed. Writing with one’s left hand is difficult even with modern writing implements, considering that this is a right-handed person’s world. Considering the crudeness of 15th-century writing tools, things were even worse for lefties in those days. And da Vinci wrote in chalk, which is prone to smearing or even being accidentally erased. However, he was able to overcome those problems by simply writing in reverse.