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Secondary dominants (V/ii, V/iii, V/IV, V/V, V/vi) apply dominant function to non-tonic chords, creating temporary harmonic emphasis. Voice-leading principles remain consistent: tritone resolution, leading-tone resolution, and smooth stepwise motion. Each secondary dominant can be extended with a seventh, ninth, or other extensions, each requiring careful voice-leading resolution to its target chord's function.
From your prerequisites in secondary dominant introduction, seventh-chord resolution, and dominant-seventh tritone voice leading, you know the core mechanism: a secondary dominant borrows dominant function from another key and applies it to a diatonic chord, and the tritone within a dominant seventh chord resolves by inward contrary motion (the leading tone rises, the seventh falls). Extended voice-leading applications take this further — systematically working through all five common secondary dominants (V/ii, V/iii, V/IV, V/V, V/vi) and their seventh-chord forms, understanding how each one's chromatic content creates specific voice-leading obligations.
The unifying principle is that voice-leading rules are function-specific, not key-specific. The leading tone of any dominant chord resolves upward by half step to the root of its target chord. The seventh of any dominant seventh chord resolves downward by step. The tritone formed between the third and seventh contracts inward. These rules apply identically whether the chord is the primary V7 resolving to I or a secondary V7/vi resolving to vi. In C major, V7/V is the chord D-F#-A-C: F# is the leading tone of G (the tonicized chord), and it resolves upward to G. C is the seventh, and it resolves downward to B. The tritone F#-C resolves inward: F# rises to G, C falls to B. This is the same pattern as G7-C (B rises to C, F falls to E), just shifted to a new reference point.
Each secondary dominant introduces a specific chromatic pitch that functions as the temporary leading tone. V/ii introduces #1 (C# in C major, pointing to D). V/iii introduces #2 (D# in C major, pointing to E). V/IV is unique — it is built on the tonic pitch (C-E-G-Bb in C major) and introduces b7 (Bb, the seventh of the chord), which resolves down to A (the third of IV). V/V introduces #4 (F# in C major, pointing to G). V/vi introduces #5 (G# in C major, pointing to A). Each chromatic pitch must be approached smoothly — typically by half step from its diatonic neighbor — and resolved correctly to the root of its target chord.
When secondary dominants are extended with ninths or other upper voices, the additional tones create further resolution obligations that follow the same logic. A V9/ii chord adds the ninth above the root of the applied dominant, and this ninth typically resolves down by step to the fifth of the tonicized chord. The practical discipline is consistent: identify which note is the temporary leading tone, resolve it upward; identify which note is the chordal seventh, resolve it downward; resolve the tritone inward; move all other voices by the smoothest available path. Once you internalize this as a transferable pattern rather than five separate rules for five separate chords, secondary dominants stop being a confusing catalogue of chromatic exceptions and become straightforward applications of dominant voice-leading logic aimed at different targets.
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