A fascinating discussion of the significance of mirror writing appears in the Lancet for December 18/25, 1999. This strange phenomenon was defined by Macdonald Critchley in 1928 as "that variety of script which runs in an opposite direction to the normal, the individual letters being reversed". Mirror writing is difficult to read, and even more difficult to write. To write it while gazing into a mirror in an effort to reverse the script is far from simple.
Mirror writing is nearly always accomplished with the left hand, and is often produced by healthy individuals, although sometimes it denotes neurological disease and may be associated with stammering or dyslexia. As a rule, mirror writing by right-handed people is the sequel of a right hemiplegia which has persuaded them to resort to the left hand. Children when they first learn to write may adopt mirror writing, but few continue the habit when they become adults.
Some left handers adopt mirror script for their own mysterious purposes. The most celebrated among these is Leonardo da Vinci, who produced most of his literary work writing from right to left in mirror script. He did occasionally write letters and notes in normal script. It has not been established whether Leonardo deliberately chose to use his right or his left hand, but he is supposed to have been originally left-handed.
Another famous mirror writer was Lewis Carroll, originally thought to have been left-handed, but who regularly used his right hand in his compositions. He wrote many mirror scripted letters to his young acquaintances for sheer entertainment.
Acquired mirror writing most commonly is the result of a cerebrovascular lesion, usually located in the left hemisphere, but does not necessarily denote a pathological state. It is extremely rare for natural left-handedness to predispose to mirror writing in its possessor.