The narrowest condition of the four, and the only one that reads no chord qualities at all. It looks at two neighbouring chords and measures the distance between their roots. If that distance is a whole tone going up, a diminished seventh goes in between, built a semitone above the first root.
Filling a step
Two roots a whole tone apart leave a gap with exactly one note in it, and the chord built on that note has a specific job: it belongs to neither of the chords on either side, so it cannot be mistaken for either, and it resolves in both directions equally well. What a listener hears is a bass line that climbs by step instead of jumping, with a chord under every step of the climb. What a soloist hears is a bar where none of the notes they were planning on are wrong for very long.
It is the least structural of the five rules and the most decorative, which is not a criticism — a turnaround that climbs rather than circles is a different piece of information to hand a listener, and this is how it climbs.
Where it fires, and where it does not
It fires in exactly one declared place on this site, and the tables below show it: the rising turnaround, twice. Between the tonic and the supertonic, and again between the supertonic and the mediant. That is the textbook use, and it is here because the turnaround that produces it was declared for this reason.
Everywhere else it reports nothing, and the pages say so instead of quietly rendering an empty table. The blues moves by fourths and fifths and one semitone; rhythm changes moves by fourths, fifths and a sixth; the other three turnarounds circle rather than climb. Not one pair of neighbouring roots in any of them is a whole tone apart going up.
Why silence is worth publishing
A rule that fires nowhere in a form is a real fact about that form — it tells you the roots are not moving by step, which tells you something about how the bass line behaves. Hiding it would make the engine look busier and make the pages less true. Every count on this page, including the zeros, is recomputed by the site's test suite before it ships.