D

d is the fourth letter of the Latin alphabet and is used in many languages that write with the Latin script. Its English name is "dee," and it appears in both uppercase (D) and lowercase (d) forms.

Origins and form: The letter D traces to the Phoenician daleth, passed to the Greek delta, then to Etruscan and Latin alphabets. The uppercase form is a vertical stroke with a curved right side, while lowercase d merges a vertical stem with a closed loop. The letter’s shape and name have varied across historical scripts, but its basic identity as the consonant /d/ has remained constant in many languages.

Phonology and orthography: In English, d represents a voiced alveolar stop /d/ and is one of the most common consonants. Its pronunciation can vary in rapid speech, sometimes realized as a voiced alveolar tap [ɾ]. In other languages, the letter name and sound can differ, but in most uses it denotes a dental or alveolar stop in common spelling.

Uses in mathematics and science: d is widely used as a differential in calculus (for example, dy/dx), to denote an infinitesimal change, and as a symbol for distance or a small variation. In statistics, Cohen’s d measures effect size. In physics and engineering, d can denote diameter, displacement, or other distance-related quantities. It also marks dimensions in geometry and databases.

Other conventions: In music, the note D represents a pitch; as a roman numeral, D equals 500. In calendars, d abbreviates day. Unicode and ASCII assign D and d to specific code points (U+0044 and U+0064).