Spelling and Hearing Secondary Leading-Tone Chords

A leading-tone seventh chord is built by stacking three minor thirds on top of a leading tone. Each minor third spans three half steps, so after three stacks you land back a diminished fifth, or really an augmented sixth away in spelling, and crucially you end up splitting the octave into four equal parts of three half steps each. That equal spacing is the whole story: the chord has no single structural root the ear can anchor to, since every note is a minor third from its neighbors in both directions. Play it on a piano and move which key you call the bottom note, and it sounds identical every time.

To spell a secondary leading-tone chord, pick the chord you want to tonicize, find the note a half step below its root, and stack three minor thirds on that note. In E flat major, ii is F minor, so the leading tone of F is E. Stack minor thirds on E: E, G, B flat, D flat, spelled as a fully diminished seventh (vii-o7/ii). It resolves by moving its bass up a half step to F while the other voices resolve normally, landing on ii. In A major, IV is D major, so the leading tone of D is C sharp. Stacking gives C sharp, E, G, B flat, the vii-o7/IV, resolving up a half step into D.

Because the chord splits the octave evenly, any of its four notes can be heard as the leading tone. The same four pitches, respelled, can resolve to four completely different chords depending on which note you treat as a half step below the target root. This ambiguity is exactly what later lets composers pivot into new keys: a single diminished seventh chord becomes a hinge, reinterpreted to launch a real modulation rather than a brief tonicization.