A Full Diatonic chord in this context is any chord in the key that has it's Root and Chord type in the scale.

Secondary chords are a type of altered or borrowed chord, chords which are not part of the key the piece is in. They are by far the most common sort of altered chord in tonal music. Secondary chords are referred to by the function they have and the key or chord to which they function. Conventionally, they are written with the notation "function/key". Thus, the most common secondary chord, the dominant of the dominant, is written V/V and read as five of five or the dominant of the dominant.

The Major or Minor triad on any diatonic scale degree may have any secondary function applied to it; secondary functions may even be applied to diminished triads in some special circumstances. (from Wikipedia)

For any secondary dominant chord, and expansion substitution for the V , its II chord can also be preceded by its II — for a II V of V .

Secondary Dominant chords fall under the Partial Diatonic harmonic principle and are labeled as such.

Here are the possible Secondary Dominant chords as defined by the above definition for both Major and Minor tonalities.

V of …

The I7 II7 III7 are possible in both major and minor tonalities. The VI7 and VII7 are possible in major tonalities only.

Chained Secondary Dominant Cycles

When several Secondary Dominant chords are used in a row and resolve up a fourth or down a fifth to a seventh chord, this is called a Chained Secondary Dominant Cycle.

These Secondary Dominant chords can be preceded by there II chords.

Things can even go further with incorporating the Minor Third Substitution Principle and doing a tritone sub for the V for a II ♭II7 .

Secondary Dominant Cycle Examples

Here are a few chained secondary dominant examples.

• Five Foot Two, Eyes of Blue
• I Got Rhythm (Rhythm Changes) (Bridge) III7 , VI7 , II7 , V7
• Scrapple the Apple (Rhythm Changes Bridge) III7 , VI7 , II7
• Jordu contains two secondary dominant cycles in the bridge.
• Stompin' at the Savoy contains a secondary dominant cycle in the bridge starting on the IV7 chord.

Diminished Chords?

°7 vs. °7♭9

Some diminished chords are really functioning as disguised secondary dominant chords. So...

When is a Diminished Chord NOT a Diminished Chord?

To determine if a diminished chord is functioning as a disguised secondary dominant write out all four notes of the diminished chord: Example C#°7 is C# E G Bb

1) Drop each note 1/2 step ( C, Eb, Gb, B ) and make each a root of a 7th chord:

C7, Eb7, Gb7, A7

2) If the next chord is a major 7 or minor 7 chord, which if it was a I chord and the preceding chord could be its V chord. Then the diminished chord is really a 7♭9 chord — A Disguised Secondary Dominant.

C7 is the V of F • Eb7 is the V of Ab • Gb is the V of Cb • A7 is the V of D and this case Dm — with Dm the II in the key of C major and we have a winner: A7 is the V of II , A7 to Dm a Secondary Dominant.

So C♯°7 is really A7♭9/C♯

Examples in Key of C

*Note: C♯°7 is the enharmonic equivalent Db°7, D♯°7 is the enharmonic equivalent E♭°7, etc... See the lesson on Enharmonic Equivalents for more information.

Additional Resources

In them spirit of TMI (Too Much Information), here are few more to consider:

• Secondary Chords

Bottom Line — when a diminished seventh chord resolves to a chord 1/2 step above its root it is functioning as a disguised secondary dominant chord.