In the previous part of this series
on music theory, we saw the relationship that exists between keys and their corresponding diatonic chords, and how these chords are built by stacking a series of third intervals on each note of the key.
In this article we’ll talk about the three types of functions that a chord can assume in the context of a key and what are the key notes that must be present for a function to be established. We’ll also see how the root note of a key can be shifted across all of its notes in order to produce a series of related scales called modes that can be used to shift the tonal center of the key away from its root note as well as a means to modulate to other keys through modal interchange.
There are three functions or roles that a chord can assume within their key of origin:
Tonic, subdominant and dominant. Each of these functions stand for how distant a chord can be from the tonal center of the key, with the tonic being totally bound to the tonal center, the subdominant being somewhat apart from the tonal center, and the dominant being the farthest away from the tonal center.
Another way to talk about functions in harmony is in terms of tension and release, with the dominant representing extreme tension relative to the tonal center, the subdominant representing medium tension relative to the tonal center, and the tonic representing zero tension or complete repose relative to the tonal center.
The chords that take on these three functions on a key are the ones built on the I, IV and V degrees of the key, and they are all major if the key is major, and minor if the key is minor.
Apart from the main functional chords of a key, there is a parallel set of chords built on the VI, II and III degrees of the key that act as their minor or major counterparts. These parallel chords are minor if the key is major and major if the key is minor.
If we analyze the common notes between a main functional chord and their parallel counterpart in their first inversion, we can notice that they share the root and the third of the main chord, with the fifth being the only note that changes. We can then conclude that these two notes constitute the essence of the chord function, regardless of whether the chord is in its main or parallel form. This fact is of the highest importance, as it gives us an unequivocal means to detect the function of any chord and the role it plays within the scope of its key.
We have seen in previous articles how a chord can be rearranged in a way that the root note is replaced by some other note as the lower voice of the chord, giving way to a different pattern of intervals between its notes, as well as a different sonority that can be used to enrich the harmony and make the voice leading between the chords more expressive and melodic. This is what we call an inversion, or an inverted chord.
When we apply the same principle to scales or keys we get the modes, which are subsets of related scales that can be obtained by rearranging the notes of the scale in a similar way that is done with chord inversions, by placing a different note as the root of the scale.
There are seven diatonic modes, one for each of the seven notes of the major scale.
The Ionian mode for the I degree, the Dorian mode for the II, the Frigian mode for the III, the Lydian mode for the IV, the Mixolydian mode for the V, the Eolian mode for the VI, and the Locrian mode for the VII.
The diatonic modes are useful to establish a more precise correlation between chords and scales by coupling each chord in a given progression to a particular mode, as opposed to the more loose connection that exists between a key and its related chords. This tighter link between scales and chords is particularly useful when it comes to improvising on top of chord progressions, aiding us to keep track of each chord more effectively, even when playing melodic content.
The modes are also used to make for more variety of key choices by extending the possibilities to seven, as opposed to only two, which is what we’re limited to when using only the more ubiquitous major and minor modes.
More advanced uses of the modes include what is called “model interchange” which consists of mixing modes from different keys in order to extend the harmony of a piece beyond its initial tonal center.
And that’s all for this third installment on our music theory fundamental series. In this article we have learned about functional harmony, and how categorizing chords according to their musical function can go a long way in helping us to give more structure and intent to our chord progressions as well as aiding us to experiment with more ambiguous and advanced chords in a more controlled way.
We also got introduced to the modes and some of their uses when it comes to songwriting and improvisation. Stay tuned for the next part of the series in which we’ll take a look at the basics of melody and rhythm.