Scales & Chords
When we're looking at chord function, we're talking about chord families, or groups of chords that play the same role in their respective keys. That means that we have to start by refreshing our knowledge on the relationship between keys and notes.
Let's begin with the basics. If a piece of music is written in the key of C, that means it's based on the C major scale (an easy one for us to use, since it has no sharps or flats). The C major scale includes the notes C, D, E, F, G, A, and B (and then it starts over again with C).
However, each of those notes also represents a chord that we can build with. So, rather than understanding the key of C as just a series of notes, it's better for us to understand it as the chords C major, D minor, E minor, F major, G major, A minor, and B diminished.
Every key is like this, composed of chords within that scale. Just as C major holds the first position in the key of C, D major holds the first position in the key of D, and F major holds the first position in the key of F. So when we're talking about chord families (in this sense), we're talking about all the chords that hold the same position in their respective keys. This is important because in each key, the role of each position is basically the same. Three of them, however, define more of the composition than any others.
Tonic Chords
The first (literally) chord we need to get to know is the tonic. The tonic is the root chord of the key. So, in a C major key, the tonic chord is C major. In every key, the tonic chord plays the crucial role of establishing the tonal center of the composition.
Musical composition is all about creating tension and then resolving it, and resolution is achieved by coming back to the tonic chord. Therefore, tonic chords are used to start and end major motifs, movements, and even the entire composition. If you do a good job creating tension, then this chord is what your entire composition will be trying to reach, and when it finally resolves, there will be goosebumps.
Subdominant Chords
So how do we go about building tension? One of the best ways to do this is to move from the tonic chord to the subdominant chord, which is the fourth chord in the key. In the key of C major, the subdominant chord would be F major. The triad of F major includes the notes F, A, and C, so it shares at least one note with the C major triad of CEG. At the same time, the root of the F major chord (F) is right between the other two notes of the C major chord, E and G.
The result is a subtle buildup of tension. It's not a lot, not yet, but it's enough to let the listener know that we're moving away from the tonal center.
Dominant Chords
As we move further from the tonal center, we increase the need for resolution, and that's where the dominant chord comes into play. The dominant chord is the fifth of the key, which in the case of the C major key, is the chord of G major. The function of the dominant chord is to bring this tension to a point where it desperately wants to resolve, to come back to that tonic chord.
So how does it do this? Look at the notes in each triad. The C triad contains the notes CEG, while the G triad is made of GBD. Again, one note is shared (the G), but now we have a B in the mix. This is a big deal. In the C scale, B is the seventh note, that which is most disharmonious with the root. So, the coupling of the G and B creates stress and tension, but in a way that still connects to the tonic chord and begs for harmonious resolution (as opposed to the B diminished chord, which is just grating). This is the function of the dominant chord, to set up the music in a way that will have the most satisfying resolution, the kind that leaves everyone feeling tingly and warm inside.
Lesson Summary
Let's review what we've learned. In musical composition, each chord plays a role depending on its position within a key. There are three of these chord families that are the most important in establishing and resolving tension. The tonic chord is the first (or root) chord of the key. It establishes the tonal center and creates resolution. The subdominant chord is the fourth chord of the key. It emphasizes motion away from the tonic chord and sets up the tension. The dominant chord is the fifth chord of the key. It brings the tension to a place that demands resolution by returning to the tonic key. And that's a musical checkmate.
