D major

D major (or the key of D) is a major scale based on D, consisting of the pitches D, E, F♯, G, A, B, and C♯. Its key signature consists of two sharps. Its relative minor is B minor and its parallel minor is D minor.

D major is well-suited to violin music because of the structure of the instrument, which is tuned G D A E. The open strings resonate sympathetically with the D string, producing a sound that is especially brilliant. This is also the case with all other orchestral strings.

It is thus no coincidence that many classical composers throughout the centuries have chosen to write violin concertos in D major, including those by Mozart (No. 2, 1775, No. 4, 1775); Ludwig van Beethoven (1806); Paganini (No. 1, 1817); Brahms (1878); Tchaikovsky (1878); Prokofiev (No. 1, 1917); Stravinsky (1931); and Korngold (1945).

It is appropriate for guitar music, with drop D tuning making two Ds available as open strings. For some beginning wind instrument students, however, D major is not a very suitable key, since it transposes to E major on B-flat wind instruments, and beginning methods generally tend to avoid keys with more than three sharps.

D-flat major

D-flat major is a major scale based on D-flat, consisting of the pitches D♭, E♭, F, G♭, A♭, B♭ and C. Its key signature has five flats.

Its relative minor is B-flat minor. Its parallel minor is D-flat minor, usually replaced by C-sharp minor, since D-flat minor, which would contain a double-flat in the key signature, is rarely used for practical composing and arranging, with a similar problem with C-sharp major. Therefore, D-flat major is quite often used as the parallel major for C-sharp minor. For example, in his Prelude No. 15 in D-flat major ("Raindrop"), Frédéric Chopin switches from D-flat major to C-sharp minor for the middle section in the parallel minor, while in his Fantaisie-Impromptu, primarily in C-sharp minor, he switches to D-flat major for the middle section for the opposite reason. Ferdinand Ries' third concerto likewise switches to D-flat major for a while for the return of the second theme in the first movement.

D-flat major is enharmonic to C-sharp major. In music for the harp, D-flat major would be preferable, not only for the reason that harp strings are more resonant in the flat position, but also because modulation to the dominant key is easier (by putting the G pedal in the natural position, whereas there is no double-sharp position in which to put the F pedal for G-sharp major).