Leonardo wrote in small notebooks using his left-hand and a technique of mirror writing (text is written from right to left). He frequently painted using his left-hand and only ever seemed to write with his right-hand when he intended the text to be read easily by others. The mirror script makes Leonardo's work on any topic very difficult to read, plus he made use of unusual spellings and abbreviations, and arranged his notes in no logical order.

Several reasons have been put forward for the use of mirror writing:

• He was left-handed and in those days of writing with pen and ink this was a particular disadvantage; as your hand crossed the page it would smudge the still wet text. Leonardo would have solved this problem with his technique of mirror writing.
• He may have been concerned about others stealing his ideas, though mirror writing seems a less than effective form of security.
• Leonardo's ideas disagreed with the teachings of the Roman Catholic Church. In particular, he discounted the idea of the Great Flood and the forming of the Earth. Again this assumes he was using mirror script as a form of security.

In the age of Leonardo, people were naturally superstitious and children were normally forced to write using their right hands, so it is unusual to find someone from this period who was so openly a south-paw.

When it comes to painting and drawing, Leonardo initially seems to have been ambidextrous, using whichever hand was most convenient. He gradually changed over to almost exclusively using his left hand. Leonardo's left-handedness has assisted greatly in identifying drawings which may be his. A right-handed artist instinctively shades with the lines going from the right down to the left, but a left-handed artist reverses this process. This can be clearly seen in Leonardo's Portrait of an Unknown Woman.