Secondary Leading-Tone Chords

A leading tone sits a half step below tonic and pulls toward it. Secondary leading-tone chords borrow that same pull for a chord other than tonic. You find the target chord, usually V or another major or minor triad, then build a diminished seventh chord a half step below its root. That root becomes a temporary leading tone, written as viio7 of the target, and it resolves up by half step into the target's root while the other voices move by the normal rules of diminished seventh resolution.

The pull is strong because a fully diminished seventh chord is built from stacked minor thirds, a perfectly symmetrical shape with no single note anchoring it to one key. That symmetry makes it sound unstable and restless in every direction at once, so when it resolves, the motion feels decisive. A dominant seventh only has one tritone pushing it forward; a fully diminished seventh has two tritones, so it pulls harder toward its target than a secondary dominant does. This is why composers often reach for viio7 over V7 when they want an especially urgent push into a chord.

Spelling one takes three steps: find the target chord's root, drop a half step to find the new leading tone, then stack minor thirds upward from there, raising notes with accidentals as needed to keep the chord fully diminished. In minor keys the raised leading tone is already sitting in the scale, so the fully diminished spelling is easy to reach, which is why minor keys favor it over a half-diminished seventh. These chords often sit right at the hinge where a passage stops merely borrowing a key for one chord and starts actually modulating into it.