"Part of the challenge wearable computer designers face is
designing usable input that preserves the flexibility users will
expect while eliminating some of the bulk. Past solutions have
involved chording keyboards like the Twiddler and compact keyboards
worn on the wrist such as those provided with Xybernaut's wearable
computers."
"IBM has also been working on this issue, and one of the
solutions they have come up with is the half-keyboard.
Half-keyboards are half the size of a standard keyboard. Each key
has two possible values, and the space key is used as a mode button
to switch between the keys assigned to each half of a traditional
keyboard."