Colemak Mod-DH is a minor modification to the Colemak alternative keyboard layout, moving the heavily-used 'D' & 'H' keys to the bottom row assignments for both index fingers. Moving the 'D' allows 'G' to return to its QWERTY placement. Moving 'H' out of the "center column" improves ease of typing 'HE', the second-most common bigram in English, now handled by adjacent fingers on the same hand in its rest position.
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Pro Much less lateral hand movement compared with regular Colemak
Some people find moving D and H to the bottom row much more comfortable (especially with flat keyboards). Curved keyboards like the Kinesis Advantage also help with lateral motion. Regular Colemak on these keyboards might be the ideal solution since then you help resolve some of the cons of regular Colemak without adding on the ones from Mod DH.
Pro Easy to learn from Colemak
No keys change fingers from Colemak (with the possible exception of Z , which is rarely used, in one variant).
Pro 'HE' easier to type compared with regular Colemak
Pro Letter G goes back to its original QWERTY position
This puts ING all on the middle row. Regular Colemak has G in the top row.
Con Regular Colemak is more standard
Mac and Linux have regular Colemak keyboard options. This isn't an issue if your keyboard is programmable through QMK, VIA, etc.
Con More optimized for ISO and ortholinear style keyboards, not ANSI
Con Index fingers jump rows more often
Con H, M, V, B change from original QWERTY position
Regular Colemak has these in their original QWERTY position. The learning curve increases somewhat in Mod-DH.
Con Breaks up ZXCV slightly
Breaks up ZXCV (moves V over one space). Could cause copy paste confusion if also using other layouts like QWERTY. For ANSI layouts the Z is also moved to a new position.