We have addressed the usability of Thumbcode first by ensuring that all states can be comfortably achieved, second by making all distinctions clear and easy to communicate, and third by classifying its states according to difficulty. For this last, the base phalanges are hard for the thumb to reach, as is the pinky when it is held apart from the ring finger (the Open and Pair closures in the unshifted case). This gives 28 ``easy'' unshifted thumbcodes, to which we assign the 26 letters, space, and carriage return.
Identifying which finger is being thumbed is progressively harder and hence less reliable for the Open, Pair, Trio, and Closed closures. The shifted closures are harder than the unshifted for the user as they require individual control of the middle-ring-pinky or trio group; it is relatively easy to open or close the whole trio. Unshifted ASCII characters are much commoner than shifted, so we assign these to the unshifted closures. We assign the commonest six letters to the easiest closure, namely Open, the next commonest four together with Space and Return to Pair, and the remaining 16 letters in alphabetical order to Trio and Closed.
The remaining ASCII characters are assigned to the remaining thumbcodes essentially according to how they are laid out on a standard PC keyboard, in aid of learnability.