A chord keyboard is another way to get a full keyboard into a small space operable with a single hand. Unlike the HalfKeyboard, a chord keyboard is potentially able to reach typing speeds comparable to the full keyboard. The downside is simple: none of your existing experience and muscle memory with full keyboards will carry over at all. In fact, it might get in the way. Which is a great practical disadvantage -- who has time to completely relearn keyboarding? And it'd be even harder if you also had to use regular keyboards while learning and potentially after learning, so you're constantly undermining your training. This is where my fascination with trying out that kind of retraining runs up against practicality.
The idea of a chord keyboard is simple: one key per finger. In its simplest form, that means you've got 31 possible keystrokes, formed by combinations of pressing multiple keys simultaneously, like chords on a piano keyboard. (Two to the fifth power is 32, but pressing no keys can't count since that's what you're doing when you're not typing, so you only get 31 options.)
Let's number the keypad for your left hand from left (pinky) to right (thumb) like this: OOOOO would mean all five keys pressed, ----- would mean none, O---- would mean just press with your pinky, and -O-OO would mean press with your ring finger, index finger, and thumb.
If you use 26 of them to mean the lowercase letters, plus one more for Space, then you have four left which each can be a "mode" for the following keystroke. To give a simple example which might not be the most efficient for typing but makes the point, consider these shift mods:
----O: Next keystroke is uppercase.
--O--: Next keystroke is a numeral, calculator key, or function key.
-O---: Next keystroke is a punctuation symbol.
O----: Next keystroke is a control key.
In each case, press the same keystroke to "lock" that mode, and then press it a third time to jump out to "default". Otherwise, it clears on the next keystroke. For simplicity, we'll assume you can switch from any mode to any mode, so if you've already pressed --O-- and then without pressing anything else you press O---- you are now in Control Key Mode, and the earlier press into Numeral Mode is ignored. Thus, each mode offers 27 possible keystrokes.
A few symbols are duplicated (* and / appear on both calculator keys and punctuation keys modes) and there's room for expansion in the control keys mode (all the combinations labelled "n/a"). This is of course just a proof of concept. Probably there are better ways to map the keystrokes to the various letters, numbers, and symbols. Besides, in practice, most chord keyboards fudge the concept a little by taking advantage of the opposable thumb to let the thumb have several keys it can press, which avoids most of the clunky mode switching.
The alert reader may already be considering comparisons to a Braille keyboard, or even to Morse code.
Could I really learn to use something like this productively, so that my hands just do what I think without me having to consider the mechanics of the process, as they do now with a QWERTY keyboard? I would just love to find out. But not enough to pay hundreds of dollars for a chord keyboard and then struggle with finding the time to practice it. Still, if it were possible, it might be more efficient than the HalfKeyboard, and even more efficient than QWERTY. Plus, you could fit a chord keyboard onto even very small devices. Imagine using a chord keyboard on a cell phone to text, for instance. If there were a standard chord keyboard for PCs so people knew it, they could chord-key on any tiny device just about as fast and efficient as on the PC. What a difference that would make, given that the biggest limit on miniaturization of devices is not computer power, not even batteries, but the human interface.