Mod-DH is a minor modification to the Colemak keyboard layout, designed to make typing more comfortable.
Colemak is a great layout, but that doesn't mean it's impossible to improve on! The idea behind Mod-DH is that by making an adjustment to the placement of a small number of keys, it is possible to gain a significant improvement in ergonomics and comfort. See this comparison of Mod-DH with other layouts.Highlights:
The mod consists of two separate parts, the left-hand "D" change, and the right-hand "H" change. It is perfectly acceptable to apply the mod to one side of the keyboard only.
The primary change is to the left hand:
1. The "Angle Mod" (see below) is applied to keys Z, X, C. This moves them one space to the left but retaining the conventional finger.
2. Three keys are relocated: D, G, B. This makes the D key much easier to type, using your index finger. The position of G is also improves, and reverts to its Qwerty placement. The B key moves from the hard-to-reach position at the bottom-middle to the new location on the top row.
There is a minor change to the right hand:
The H and M keys exchange places. This makes H easy to type, being the more common character than M. In addition, since HE is the second most common bigram in English, it makes it more comfortable to type very common words such as "the", "then", "where", etc.
If you are currently using Qwerty and looking for a better layout, then both standard Colemak and Mod-DH are excellent choices. The change from Qwerty is not easy and can take some time, but is worthwhile in the end. If you start with Colemak but find the frequent reaching for D, H and G unsatisfactory, then Mod-DH is for you!
For many people, nothing! But others who have tried Colemak have identified flaws, and have found the transition from Qwerty frustrating as a result. Some have even given up on Colemak and looked to other layouts instead. I believe that those who experience problems with Colemak need not give up on it. Mod-DH was created to fix the most common complaints, and to give a more comfortable Colemak experience for those who want it.
The most commonly identified flaw is Colemak's placement of the D and H keys. As the 8th and 10th most common letters in English, they should ideally be in easy-to-reach positions. Colemak places them on the centre column, which means a lateral motion of the hand is required to access them. If you try this motion frequently, you'll probably find it is somewhat less comfortable than simply moving the index finger down, to where Mod-DH places those keys. The H-E combination is often cited as a common English bigram that is unduly awkward with Colemak. If you think it's easier for the index finger to reach down one key to the bottom row than to the centre column, then you'll find Mod-DH more comfortable to use.
My own experience is that I used standard Colemak for several months, but was somewhat unsatisfied with the frequent reaching for the middle of the keyboard to type D and H, and to some extent G. But I stuck with it, being reluctant to change an established layout and thinking Colemak's other benefits would outweigh its drawbacks. And to be fair, for the most part they do. But I eventually came to realize that in fact there is no need to compromise: By applying Mod-DH, it is possible to retain Colemak's best features, while placing some common letters in easier to access locations and reducing the need to reach for the centre column. Within a couple of days using the left-hand "D" mod, I found it to be a noticeable, significant improvement over standard Colemak. And when applying the right-hand "H" mod, common English words containing HE (the, then, where…) become almost effortless.
For more information, see this Guide to the Angle Mod.
The Angle Mod is a good innovation, but in its default application it assigns the rare V to one of best locations on the keyboard. Mod-DH fixes this by assigning D to that key instead. An important feature of Mod-DH is that on standard keyboards, use of the Angle Mod idea is intrinsic to the layout design.Curl-DH [DbgHk] variant.
The mod works best on keyboards which have good support for the "Angle" mod i.e. ISO keyboards, and symmetric ergonomic keyboards. It can also be applied successfully to ANSI keyboards with an adjustment. Let me know if you have other suggestions for implementing on an ANSI keyboard.
Mod-DH is now also supported in DreymaR's Big Bag of Keyboard Tricks.
The keyboard layout and the implementations are available under the public domain. You have the right to freely use it for any purpose, without any conditions. Attribution is strongly encouraged, but not required.
The downloadable software is provided "as is", without warranty of any kind, express or implied, including but not limited to the warranties of merchantability, fitness for a particular purpose and noninfringement. In no event shall the authors or copyright holders be liable for any claim, damages or other liability, whether in an action of contract, tort or otherwise, arising from, out of or in connection with the software or the use or other dealings in the software.