Language expression is known to be dependent on attributes intrinsic to the author. To date, however, little attention has been devoted to the effect of interfaces used to articulate language on its expression. Here we study a large corpus of text written using different input devices and show that writers unconsciously prefer different letters depending on the interplay between their individual traits (e.g., hand laterality and injuries) and the layout of keyboards. Our results show, for the first time, how the interplay between technology and its users modifies language expression.