Lyman Lipke here, with your semi-regular dose of jazz knowledge. Today, we’re going to be taking a look at a concept called “secondary dominant chords”. A quick definition, in any given key, a chord that is dominant (contains a major 3rd and a minor 7th) that isn’t the V chord, is called a secondary dominant. Confused? All right, here’s a quick example. In the key of C, you might see an A7, an E7, a D7, a C7 or an F7. Those would all be considered secondary dominant chords. If you’re feeling especially spicy, you might see chords like Bb7 and Eb7 in the key of C. G7 is not a secondary dominant in the key of C.
Now the question we have here, is why do we need to know this? And that’s completely valid. Secondary dominants are very common move in jazz, and if we study how these chords are used, and how they interact with diatonic chords and other secondary dominant chords, we can make more informed decisions when improvising.
For the examples today let’s use the progressions I-vi-ii-V and iii-vi-ii-V.
This is a standard way to play I-vi-ii-V-iii-vi-ii-V. Let’s change the vi chord to a secondary dominant chord.
Ok, interesting. Again, why would we do this? My primary reasoning and thought process is that resolving downwards in half steps is stronger harmonically than resolving in whole steps. It also opens the door for more interesting melodies.
Another point of interest is the iii chord. E minor contains the notes E and G. C major (the I chord) contains the notes E and G as well. E minor’s sound doesn’t conflict with the characteristic of C major. So this means in a lot of situations, we can substitute the I chord with the iii chord. And the movement of E minor 7 to A7 is a simple ii-V movement. We can use this as another justification for turning the vi chord into a secondary dominant.
Now, we could go one by one, changing each minor chord into a dominant chord, but I’ll let you do that on your own time. Let’s see what happens when we change each minor chord into a dominant chord, and how those 3rds and 7ths interact with each other.
Interesting. If we’re looking at the 3rds and 7ths from the dominant chords in each bar, respectively, every 3rd resolves down to the next chord’s 7th, and every 7th resolves down to the next chord’s 3rd. We can use this information when creating lines to better connect the sounds of the chords.
Now, there are more secondary dominants that can show up in any given key, but we’ll address those at a later time. In the meantime, practice coming up with lines over variations of the I-VI7-II7-V7 and III7-VI7-II7-V7 progressions giving each chord 2 bars, then 1 bar, then half a bar. Hopefully this helps demystify all of the various dominant chords you see when looking at a jazz chart. Get familiar. Happy practicing.