A related one-handed, eyes-off way to text fast is with the Twiddler chording keyboard, used by the wearable computing people, circa turn of the century: https

A related one-handed, eyes-off way to text fast is with the Twiddler chording keyboard, used by the wearable computing people, circa turn of the century: https://en.wikipedia.org/wiki/Twiddler https://en.wikipedia.org/wiki/Twiddler Of course that, too, requires special training to use. And harder to integrate into a smartphone design than 2 side buttons. (Maybe turn the back of an iPhone-like form factor into a chording keypad, with physical buttons or some other tactile affordance?) Brad Rhodes at MIT would have a Twiddler in a pocket, a small Private Eye brand HUD on the corner of his hat brim, and a lunchbox-sized PC on a shoulder strap. He could be typing and looking up information while he was talking with someone or in a meeting, without them necessarily being able to tell. (Though, if someone did notice use of the wearable computer, they might not say anything anyway, since -- as he eventually realized -- some people who hadn't gotten the demo thought that the box with the cables coming out of it and going into clothing might be a medical device.)