D. Voiced alveolar dental stop. You use your voice and soft palate to make the sound, make your breath stop against your teeth. Leave your larynx out of it and you make a T. D was sometimes spelled with a T in later Old English, and it would occasionally appear as the letter Eth which looks like this: Ð and this: ð. Eth is kind of a cross between a Þ (thorn, TH) and a D. A th sound with a little D flavor. Eventually the Ð and Þ became interchangeable leaving the D to stand alone, exiled in wretchedness.
Make a thorn and point it at a reverse thorn. A thorn in a mirror. Let them keep in touch, they are very close.