Recent research has found that children reverse mainly the left-oriented characters when writing from memory (e.g. they write [iota] and [epsilon] instead of J and 3). In order to obtain an objective definition of the left-orientation of a character, the ratings of the level of left-orientation of all the asymmetrical capital letters and digits by 142 adult students was analysed in Study 1. Study 2, on 298 five-six-year-old children, examined an immediate prediction of Study 1, namely that the children reverse mainly the digits that the adult students have rated left-oriented. Other predictions, both of the posited representation of the writing during the reversal stage and the neurological process of mirror generalisation, were verified: the simplicity of the representation of the symmetrical digits 0 and 8 makes incorrect writings very rare; the mirror generalisation, which operates only in the left-right direction, makes other transformations (inversion or 180° rotation) very rare. Finally, the explanatory power of some putative individual factors of reversal (e.g. writing with the left hand) is shown to be far lower than that of the left-orientation of the characters.