What were/are the reasons for putting Space and Return on the left thumbs?

[jeffsf]

Trying out the Space and Return on the left thumbs is a struggle for me to retrain my right-thumb habit. Rather than simply give up and swap to the right, I'm wondering if there is a good enough reason to keep them on the left.

"Only because I was a left-thumb spacer out of habit" is a completely acceptable answer if that was the case.

[urob]

Great question! In parts, it's what I'm used to. But while switching up keyboard habits makes for a few really awkward days, it's also pretty quick in my experience to get used to (I not so long ago changed from GACS to GASC home-row mods, and couldn't image otherwise now).

The deeper reason is that it's the only solution I could come up with to the following constrained optimization program:

1. I only want 2 keys for each thumb to minimize lateral motion
2. smart-shift should be the tap & hold-action on one of the two home thumbs
3. space should be the tap-action on the other home thumb
4. enter should be the tap-action on the secondary thumb adjacent to shift
5. smart-num should be the tap & hold-action on a thumb
6. The nav layer should be the hold-action of one of my home thumbs (it's my most frequently used layer)
7. Layer activators should be opposite of the primary keys of the activated layer (Miryoku-style)
8. The navigation keys should be on the right side (works great in Vim)
9. The numbers should be on the left side (so I can use them for Excel or Sudoku while navigating a mouse with the right hand)

Having space on the left home thumb and shift on the right home thumb is the unique solution to 1-9. In fact, the system is over-identified, so it's somewhat fortunate that there is a solution at all.

Proof 1

10. 1-5 imply that smart-num has to be the secondary thumb adjacent to shift
11. 7, 9 & 10 imply that smart-num has to be the secondary thumb on the right side
12. 10 & 11 imply that shift has to be the home thumb on the right side
13. 1, 3 & 12 imply that space has to be the home thumb on the left side
14. 4 & 13 imply enter should be the secondary thumb on the left side

Proof 2

16. 6 & 8 imply that nav has to be the hold-action of my left home thumb
17. 2 & 14 imply that smart-shift has to be the tap-action on my right home thumb
18. 1, 3 & 16 imply that space has to be the tap-action on my left home thumb
19. 4 & 18 imply enter should be the secondary thumb on the left side

(sorry for going full nerd here, I show myself out 🤪)

[urob]

As you can see, 8 & 9 are the import inputs that pin down which keys are assigned left & which are assigned right. If they don't apply to you, you could try mirroring the entire layout. But if some of the other inputs apply to you, this may require swapping lots of keys, not just shift & space.