The Latin D's sound history, back through Phoenician dalet and Greek delta, is covered elsewhere; this entry follows the letter's outline. Dalet was drawn as a simple triangle, meaning door, and Greek delta preserved that triangular shape almost unchanged, giving classical Greek its familiar angular capital. Etruscan and Roman scribes, working in a script built more from curves than the Greek monumental alphabets, rounded delta's triangle into a bow set against a straight vertical stroke, producing the D shape already fixed in Roman inscriptions by the classical period. That silhouette has proven remarkably stable: the lowercase d that emerged through Roman cursive and later uncial handwriting simply extended the vertical stroke upward into an ascender while keeping the bow low, and that structure passed into the Carolingian minuscule of the 9th century essentially unchanged from the capital's own logic.
D also serves as a Roman numeral, worth five hundred; unlike I, V and X, its numeral value is not explained by any tally-notch ancestry, and one common account derives the symbol as half of an older sign for one thousand split along its vertical stroke, though, as with several of the larger Roman numerals, the derivation is not settled with certainty.