Voice Leading with the Dominant Seventh

A V7 chord is built on the fifth scale degree and adds a minor seventh above the root, so it has four distinct tones: root, third, fifth, and seventh. Two of those tones are unstable and each has only one place it can go. The third of the chord is the leading tone, and it wants to rise by half step to the tonic. The seventh of the chord is a dissonance against the root, and it wants to fall by step to land on the third of the tonic chord. These two voices move in opposite directions, the leading tone up, the seventh down, which is exactly what keeps the independent lines from running in parallel motion.

Follow a full resolution. A complete V7 has all four tones present: root, third, fifth, seventh. When it resolves to a complete I chord, the root of V moves down to become the root of I, doubled, while the third and seventh resolve as described. But doubling the root of I while also keeping four full tones moving correctly often leaves the resolved tonic chord incomplete, missing its fifth, because the leading tone and seventh both claim notes that are already covered by root and third. That trade-off, three correctly resolved tendency tones against a sometimes-incomplete final triad, is normal and accepted.

This same logic returns later whenever a secondary dominant seventh resolves. The chordal seventh and the raised leading tone of that secondary dominant behave identically, stepping toward the chord it momentarily points to, which is why mastering this one case makes every later tonicization easier to voice.