Computational layout design for keyboards with multi-letter keys

Keyboards with multi-letter keys (i.e., a key corresponds to multiple letters) have been commonly used on small touchscreen devices to mitigate the problem of tapping tiny keys with imprecise finger touch (e.g., T9 keyboard). We have proposed a computational approach to designing optimal multi-letter key layouts by considering three key factors: clarity, speed, and learnability. In particular, we have devised a clarity metric to model the word collisions (i.e., words with identical tapping sequences), used the Fitts-Digraph model to predict speed, and introduced a Qwerty-bounded constraint to ensure high learnability. Founded upon rigorous mathematical optimization, our investigation led to Qwerty-bounded T9-like (i.e., 3 × 3) layouts optimized for both clarity and speed. A preliminary user study showed promising performance of such keyboards.