We all know the frustration involved with shifting from mousing to typing. It is therefore not surprising that Douglas Engelbart - the inventor of the mouse - intended his device to be paired with a one-handed keyboard in addition to a regular QWERTY keyboard. His "keyset" was composed of five keys shaped like white piano keys, and could be used in conjunction with three mouse buttons. The typing scheme mapped the binary number represented by the keys to letters.
Whether due to the need to retrain, the increased difficulty in training, or, perhaps, a cost-to-benefit ratio to low to justify, chorded keyboard never caught on. Nevertheless, the allure of increased efficiency is strong.
The keyboard follows Engelbart's by mapping binary numbers to the alphabet. The numbers are represented by pressing combinations of five keys, allowing for 25-1 combinations. Numbers 1 through 26 are lower-case Roman letters; the remainder are the following special characters:
31 is thus far unused.
A letter is typed on release rather than key press to alleviate timing issues.