Learnability would ideally be addressed by using one or another well-known arrangement of the letters, e.g. alphabetical as on the Handykey Twiddler or QWERTY as on standard keyboards. Unfortunately the well-known arrangements are far from optimal with regard to usability.
However by careful design it is possible to combine a near-optimal arrangement with a reasonable degree of learnability. For the latter we have made use of four techniques: a mnemonic system for the ten commonest letters, alphabetical order combined with regularity of subsetting for the remaining letters, a close match to customary PC keyboard layout of numerics and punctuation, and assignment of pairs, namely parentheses, square brackets, curly brackets, and slashes, to pairs of base phalanges.
The rationale for this assignment is as follows. The ten most commonly used letters, along with Space and Enter, are assigned to the six easy phalanges of the easiest two closures, namely Open and Pair. Within those ten letters, the order is chosen for its mnemonic value, forming the words ``tea,'' ``sin,'' ``oh,'' and ``dr'' (doctor). (Actually d is slightly less common than l and c, but moving d up ahead of l and c results in an attractively regular structure for the remaining 16 letters that should make it easy to learn.)
For the Trio and Closed closures, the thumb can reach the pinky more comfortably, and all four fingers are used in assigning the remaining 16 letters to the two easy phalanges. The layout of these letters in the alphabet, namely *bc**fg**jklm**pq***uvwxyz, leads to a natural and therefore easily memorized grouping: bcfg, jklm, pquv, wxyz.
The numerics and - and = are laid out on the hard phalanges in the easy
closures, in the order found on the keyboard. The remaining lower-case
glyphs continue the keyboard order on the hard phalanges on the hard
closures, followed by
at the end (which is happy to be next
to /).