The Key of D, from the Melody

D major harmonises to D – Em – F#m – G – A – Bm – C#°; D minor to Dm – E° – F – Gm – Am – Bb – C. A son in this key mostly alternates the tonic and the dominant; a ranchera adds the subdominant; a bolero reaches for the submediant and the supertonic on its way home.

In the melody section this instrument is the melody section's written line — several players bowing one part together, so intonation is a group problem before it is anybody's solo. A chord on a bowed neck is really a choice of double stops drawn from these tones, plus the arpeggio a bow can walk through the rest — and a section playing in unison has to agree on which one, because two answers at once is a chord nobody wrote.

Learn the tonic, subdominant and dominant in D until the hand goes there unasked, and the repertoire in this key is largely playable before anybody names a form.