The Key of Bb, from the Melody

Bb major harmonises to Bb – Cm – Dm – Eb – F – Gm – A°; Bb minor to Bbm – C° – Db – Ebm – Fm – Gb – Ab. 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 Bb until the hand goes there unasked, and the repertoire in this key is largely playable before anybody names a form.