The function of a secondary dominant is to emphasize a triad other than the tonic in a manner similar to how the natural dominant in the key reinforces the tonic. If a secondary dominant is only used to temporarily emphasize a triad, it is referred to as a tonicization. However, if this triad is then established as the new key center (by means of a cadence for instance), the use of the secondary dominant can then be thought of as a step towards a modulation.
1. Finding Appropriate Triads for Emphasis by a Secondary Dominant. In the following brief phrase, we have composed a simple progression. In the first measure we have confirmed the key region for the listener by emphasizing the tonic through the use of its dominant. In bar two, this is followed by two pre-dominant triads (IV and ii), concluding with a half-cadence on the dominant in the third measure. We have a number of places in which to insert secondary dominants: the dominant in the first bar, either of the pre-dominants in bar two, or the dominant in bar three. Since we want to avoid interfering with the establishment of the key in bar one, we can choose any of the subsequent chords in the final two measures.
2. Inserting Secondary Dominants. Two principal techniques that we can use to bring secondary dominants into play is to either:
a. Insert a chord within the context of the phrase.
b. Alter one of the chords chromatically.
In the example below we will demonstrate both of these methods. When creating a secondary dominant, we have to momentarily imagine the chord to be emphasized to be a 'tonic'. Since the IV chord in Bb is Eb, our secondary dominant is the same as the dominant (or dominant seventh) in Eb major (Bb, D, F, Ab). The I chord in Bb is, obviously, similar to this chord and we have therefore chosen this region to insert a chord between the I and IV (through the addition of the Ab). The resolution of a secondary dominant should employ the same correct voice leading concepts as applied to any dominant in the Common Practice.
Our next secondary dominant will be used to emphasize the final V chord of the phrase. Note that since the V chord in Bb is F, the secondary dominant needed here will be the same as the dominant in F major (C, E, G, Bb). The chord existing in this place in the original phrase is the ii7 in Bb (C, Eb, G, Bb) and therefore we only need to chromatically alter the Eb to E natural in order to create the appropriate sonority.
3. Distant Relations. Of course, a composer may wish to emphasize a triad that lies further from the key center than other close diatonic chords. However, in order to articulate these desired secondary dominants, increased chromaticism will be needed. In the following example a secondary dominant is inserted to emphasize the mediant in bar two: note the need for two chromatically altered notes to correctly spell the chord. In the following bar a Neapolitan (bII) is being inserted as a terminal pre-dominant chord. Here the composer wants to introduce another secondary dominant to tonicize the Neapolitan itself, and in so doing, requires three chromatically altered tones.
