Chord Theory - Adding Sevenths
Continuing our discussion of chords, we will now move on to adding sevenths to chords. Remember that the four basic triads form the basis of all other chords, so all you need to do is use them as the starting point to build all of the other more complex chords.
The dominant seventh chord starts with the major triad, root, third and fifth, and adds the flatted seventh from the scale to complete the chord. For C7 then this would give us C, E, G, and Bb. Notice that in the harmonized scale, that this chord occurs on the fifth degree of the scale, and only here. For that reason, dominant seventh chords are often indicators of a key change.
Degrees two, three and six of the scale are minor sevenths. To build these, you take the minor triad, and like the dominant seventh, add the flatted seventh of the scale. For Cm7 then, we would have C, Eb, G and then Bb. So how do you determine key here? If there is no other context based on the surrounding chords, you will usually treat the m7 as a II chord. That said, try the other two keys the chord occurs in, as they can give an entirely different color to your ideas. Remember, improvising is about experimenting, not rules.
Major seventh chords occur on the first and fourth degrees of the scale. Once again, we take the major triad, but this time add the seventh of the scale. For Cmaj7 then we, have C, E, G, and B. Try both scales that these occur in also to see how they sound against the chord.
The last chord type that we have in the scale is the half diminished chord, also written as m7-5. Here we use the diminished triad as our base chord and add a flatted seventh to complete the chord. Like the dominant chord, this is in only one place in the scale, but keep in mind that this is the II chord in the minor keys, so that has to be a consideration as you use the "key center technique" that was discussed in a previous article.
There are of course other chords beyond these and we will get to them in future articles. Familiarize yourself with these first, so that you can build them in all twelve keys without hesitation. As always, the sooner you can work them into your playing, the faster you will be able to call them up when you need them. Once you have practiced them, you will be able to recognize them when you hear them played by other players, or when you hear them in your head as you create your own ideas. This is just another step toward playing what you hear.