Secondary chords II - leading tone chords
Using diminished chords as a secondary harmonies
Diminished triads and seventh chords are associated with the leading tone, having a dominant function that pulls us to the tonic chord.
Since viio (with or without a seventh) is also a dominant function chord, it too can function as a secondary chord like a secondary dominant. In this case it works as a secondary leading tone chord rather than a dominant, with a root one half step below the chord of resolution, sounding like a viio - I progression.
Since these chords are diminished, they require more accidentals than dominant chords. There may be both raised and lowered notes used in making secondary diminished seventh chords in any given key.
For example, we can tonicize vi in the key of B-flat with a secondary diminished seventh chord, viio7/vi. Let’s spell it out. Remember this is a leading tone chord, so this time we will find the root of the secondary chord a half step below the scale degree we are trying to tonicize. First, let’s find the sixth scale degree in the key of B-flat. That is the note G. We need to build this secondary diminished chord on the leading tone to G, which is F#. This one needs to be a fully-diminished seventh chord, so all the notes will be a minor third apart: F#, A, C, and E-flat. Notice that this chord contains both a flat and a sharp. The F# will be an accidental, and the E flat is already in the key signature.
Although we don’t use them often in music, it is common for these secondary diminished chords to require double flats or double sharps, especially the fully-diminished seventh chords, which are the most commonly used type. Here are a couple of examples to think about.
First we will spell out the viio7/IV in A-flat major. The fourth step of the A-flat scale is D-flat. The leading tone of D-flat is C. Therefore we need to spell a C fully-diminished seventh chord: C, E-flat, G-flat, B double-flat. Secondly, we will spell out one in a key with sharps: viio7/iii in F# major. The third step of the F# scale is A#, and the leading tone to A# is G double-sharp (make sure you don’t call it A!) Therefore the notes in a fully-diminished seventh chord with the root G double-sharp are: G double-sharp, B sharp, D sharp, and F sharp. Always take your time to make sure you have the correct root for your leading tone chord: it needs to be one letter name and exactly one half step below the name of the scale degree you are tonicizing.
Again, do not respell notes enharmonically to make the chords simpler, since that will change the root of the chord and not agree with the Roman numeral function.
Resolving secondary fully-diminished chords
As with secondary dominant chords, secondary leading tone chords should be resolved just like you learned to resolve diminished chords in diatonic harmony: leading tones should be resolved up to their tonics, and sevenths, if present, should resolve down by step. Parallels should be avoided. Secondary diminished chords also follow all other rules that we learned when using them diatonically, such as which notes may be omitted or doubled.
When resolving a fully-diminished seventh chord, a quick rule is to resolve the leading tone up, and all the other notes down (by step.) This will result in the doubling of the root note of the triad being tonicized, which is usually desirable. Remember that since there are no perfect fifths or octaves in a fully-diminished seventh, there is no way to have parallel fifths or octaves when resolving this chord. It is therefore a very easy chord to use.
You can also resolve the chord by moving the third of the fully-diminished seventh chord upwards instead (all other voices move same as described above), which will result in the third of the chord of resolution being doubled. Depending on what scale degree this note is, and what sort of chord quality exists, this may or may not be as desirable as the above root doubling.
Using half-diminished sevenths as secondary chords
When using a half-diminished seventh as a secondary leading tone chord, there is the possibility of having parallel fifths, since there is a perfect fifth in the chord between the third and the seventh. Thus it is important to pay special attention to the resolution of these half-diminished types. Remember, there are no new rules for resolving these chords just because they are secondary. The resolution works exactly as it would in a diatonic progression. If you need review on resolving these chords, please review the section on diatonic seventh chords.
Using diminished triads as secondary chords
Using diminished triads as secondary chords works just like using the leading tone triad in a diatonic progression. In other words, its use is somewhat limited. This triad, viio, is not often used diatonically in root position. It is most often found in first inversion, and this is the way it will be found used as a secondary chord. This triad usually has the third doubled – that tone is not part of the tritone interval between the root and fifth, and it helps to emphasize that note instead of one of the members of the tritone. Also, the leading tone can never be doubled, since that would create parallels when they both resolved to the tonic. This holds true for the secondary leading tones as well.