D as a Scale Degree

D sits a whole step above C and is the second degree of the C major scale, called Re in movable-doh solfège regardless of which key a piece is written in, since solfège names the function of a pitch within its scale rather than the pitch's fixed frequency. In the key of D major itself, however, D becomes the first degree, Do, illustrating the core distinction between the fixed letter-name system used throughout these entries and the movable, function-based solfège system Guido of Arezzo's eleventh century mnemonic eventually became.

D major and D minor both hold a substantial place in the Western concert and liturgical repertoire, and D is also the pitch most commonly used for open, unstopped strings on several standard orchestral and folk instruments, a practical consideration of instrument construction and tuning that has nothing to do with any esoteric correspondence and everything to do with the physical length and tension a given string can be built to hold in tune.