Sevenths and secondary dominants

Sevenths and secondary dominants give Bach's harmony its drive. They also come with strict rules about where they go next.

Sevenths

• The 7th resolves down by step and is never doubled.
• Non-dominant 7ths (ii7, IV7, vi7) must be prepared: the 7th sounds in the same part in the chord before.

Inversions of V7

Inversion	Bass note	Usually goes to
V6/5	The leading note	I, with the bass rising
V4/3	The 5th	I or I6
V4/2	The 7th	I6, with the bass falling

• A complete root-position V7 usually resolves to an incomplete I, with a tripled root.
• Leaving out the 5th of V7 lets you resolve to a complete I.

Secondary dominants

A secondary dominant is the dominant (or dominant 7th) of a chord other than the tonic. In D major, for example:

• E7 or G#ø7 goes to A
• B7 goes to Em
• F#7 goes to Bm
• D7 goes to G

Two rules:

• It must resolve to its target chord.
• The chromatic raised note rises, and the 7th falls.