d is the 11th most frequent letter [1], so there are many other letters vying for the 8 main home row positions. However h is 9th, it's a good candidate for a better position, since it occurs in the two most frequent bigrams (th and he).
Since backspace is used far less than frequent letters by competent typists and enter is also relatively infrequent, it is probably best to put something like e on the thumb cluster, so that h can be on the home row.
Agree on both accounts. I have 'd' on top row mid finger, right above 'h' on home row mid finger. 'e' on vowel hand thumb.
The point about frequency of 'd' being too high is with respect to having it on the index bottom row (as in the dh variant), because of the curling gesture it incurs. The index finger is tricky because being a long finger, it is comparatively better to extend up than to curl down (assuming your wrist is neutral or slightly raised), but top row index position will usually find bigrams with mid finger home row, making it a scissor (qwerty 'dr') and uncomfortable. Given it's reign over 6 keys, it is better suited for less-frequent letters on the non-home positions. 'd' would be borderline acceptable in terms of frequency, and for reducing incessant curling, inner column center row (qwerty 'g') is a better placement -- this is what dvorak, maltron, and rsthd opted for (but keep in mind this makes it more prone to the lateral stretch problem). But the better choices are from the 'mfpgwybv' pack, and perhaps 'c' to a lesser degree due to its frequency.
The index finger is tricky because being a long finger, it is comparatively better to extend up than to curl down (assuming your wrist is neutral or slightly raised)
Ah, thanks, that makes sense. I guess I have that issue less, since I use a contoured keyboard and curling the index/middle fingers is pretty comfortable.
Interesting, I find that curling the index finger is the most comfortable way to reach any key that isn't on the home row. I guess it comes down to how you have your hands positioned. I use a standard column staggered layout and have a wrist rest that sits about 1.5cm below the keys, what do you use?
I come from Microsoft sculpt where the wrist rest is built to raise your wrist higher than the finger tips (reverse tilting). If you bend your wrist down (i.e. toward the desk), then your fingers naturally curl down, whereas if you raise your wrist up (like on the MS natural), then your fingers are naturally more extended. From an ergonomics perspective, it is better to have your wrist neutral or slightly raised than to have them bent down. In that case, the top row typically requires less effort than the bottom row (particularly when reverse tilted). Now whether or not that's more comfortable also depends on how often do you encounter bigrams like 'dr' -- or worse, 'cr' -- on qwerty, where you have to extend index on rows above the middle finger (these are the 'half' and 'full' scissors, respectively, in layout analyzers). The discomfort of top row index often comes from these type of scissor bigrams and is alleviated when it's possible to also extend the middle finger slightly at the same time. Curling index finger is a move more independent of middle finger placement (but that doesn't mean less effort/stress on the tendon, particularly for wrist up folks). In other words it's possible for a key to both be more comfortable and incur more effort/stress at the same time.
I'm not sure, from the description of your wrist rest arrangement, if you are in the 'wrist up' or 'wrist down' camp. But certainly for wrist down folks, I can imagine the bottom row being more comfortable for the index.
Colemak mod-DH puts D and H right below the home row so they are easy to reach with your index fingers. I'd say that's the most reasonable way to go about it without going full custom.
For me it doesn't matter much. Since I have a contoured keyboard nearly all alpha keys are easy to reach with my hand on the palm rest (not hovering). But I'd rather have h on the home row as well, since it reduces finger movement for frequent bigrams like th. I have backspace on one of my thumb keys, but I don't use it frequently, to it'd be more optimal to put a letter like e there, so that h can move to the home row. So even though I use Colemak-DH, I am experimenting a bit with other layouts like Maltron since I don't believe Colemak it necessary among the best layouts, it just gained quite a lot of popularity. Well and it's fun to rewire parts of your brain :).
It's a bit of a shame that layouts are high effort. Given a large enough budget, you can try multiple ergo keyboards. But the cost of learning a layout is so high that people experiment only little with it (if at all), so people continue to use QWERTY or if they learn something else it would be Colemak now or Dvorak 20 years ago. Of course Colemak and Dvorak are large improvements over QWERTY, but they may also be local optima.