Any theory why H on thumb scores best?
Maybe due to being the most frequent bigrams and trigram. Split the trigram THE. This has 2 consonants and 1 vowel. To balance the rest of the keyboard, 1 consonant--the middle one--is moved to thumb.
moreover Maybe these algorithms don't punish thumb - finger bigrams. I saw something in Opt that penalize thumb - index. But we can assume T E goes on middle finger, which is not affected by this penalty. So it seems middle finger is more premium location than thumb. If E is on thumb, then best place for TH is roll inward. Noting that in some algorithms rolling is less optimal than alternating fingers, especially if alternation involves thumb.
RSTHD uses the same hand for all 3 letters of the trigram most common trigram THE. which works for him since he prefers 2 and 3 keys long sequences.
(notably RSTHD is yet another layout that divides into vowel and consonant districts. also it has space on the vowel side, just like what we've discovered earlier.)
Actually studying my digram chart (http://www.shenafu.com/digrampop.html
) , I think I see (partly) why H works best at thumb instead of other letters. There is a parallel amongst the four most common bigrams, including the space character.
1. The top 4 digrams are made of 4 different characters. So the thumb keys should nominate these as highest priority.
2. H is in the exact same situation as space, relative to T and E.
3. Putting T or E on thumb would imbalance the other bigram. ex. if E is on thumb, then TH must be typed with fingers. conversely, T on thumb means HE is typed with fingers.
4. If H is on thumb, both bigrams score better because thumb is involved for both.
5. Just realized that any finger to thumb is really an inward roll. So if H is on thumb, ideally it should on the same hand as T to make TH an inward roll. on the contrary, HE on the same hand is an outward roll. Same deal with space (instead of H).
6. Once you put more letters on the thumb, H falls out of favor. for instance, 3-letter thumb clusters are optimized for I_RN. see previous post for such a layout: http://shenafu.com/smf/index.php?topic=89.msg835;topicseen#msg835
. (Amazingly vowel and consonant districts are still maintained.) This seems to agree with my assessment that finger-thumb is inward roll. consider bigrams like: ei, ai, oi, tr, pr, dr, fr, gr, cr.