The D guitar is in effect a standard scale 12 fret to the body guitar with two extra frets at the nut and tuned a tone lower.
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12 fret guitars have always been known to have a special sound, very centred and musical. Not because the neck is shorter, rather because the bridge is moved back on the soundboard and away from the sound-hole. It's this smaller area between the bridge and the tail of the guitar that gives the 12 fret sound.
My D guitar, being in effect a long scale 12 fret guitar, has this distinctive sound.
Putting a capo at the second fret gives standard scale, standard tuning and standard string tension so has the sound and feel of a standard 12 fret guitar.
Taking off the capo puts the D guitar one tone down from standard and on a long scale. This makes some keys easier (such as F and B flat), handy for accompaniment, and gives a majestic depth and sustain.
My original D was based on a Model 1, later Ds have all been versions of the Martin Simpson Signature Model. I plan D versions of the New World and Model 4 Dreadlought guitars in the near future.
Apart from the longer neck and re-positioned bridge, D guitars are built to the same specifications (and have the same options) as standard guitars.

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Martin Simpson Signature model Mark 2 D guitar
Specifications
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Scale
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725mm (28.5") |
| Soundboard |
European, Adirondack or Italian spruce |
| Back and sides |
African Blackwood, Brazilian rosewood, Madagascar rosewood or Indian rosewood |
| Neck |
Brazilian mahogany, Cuban mahogany or Madagascar rosewood |
| Fingerboard and bridge |
Ebony |
| Trim and body shape |
As for Martin Simpson model, Model 1 or New World model |
| Pickup (optional) |
Highlander |
| Tuners |
Gold plated Gotoh 510s (black buttons optional) or standard Gotohs |
'The D must stand for divine' - Michael Nouri
'...the guitars are thriving. The Model D got a genuine lost-for-words reaction from one recent visitor' - Dougal Campbell