Having studied the degrees of a key and their function, how to build chords from them, and how to use the latter within specific progressions and cadences, we'll now further your harmonic vocabulary by looking at how to replace some of these chords for different ones.
Added tone chords
A common method for making variations to a chord is to replace it with a richer form of the same chord. I have already mentioned in a previous article that you can always add a fourth note to a triad to convert it into a seventh chord. But you can still add more tones to these chords. Generally speaking, the way to achieve this is to add additional thirds, always within the key of the song. They can sometimes be altered, especially when it comes to passing chords (refer to article 8) or to make a modulation (a concept I'll discuss later on).
In this case we are talking about superstructures. But you can also add an additional major second to the basic triad, in other words, the sixth of the chord's root. This is called a sixth chord. Do note however that a major sixth chord can also be considered as the first inversion of the seventh chord built from the relative minor.
Below is an example of a C6 as the first inversion of Amin7:
Degrees that share a same function
As we've seen in the previous articles, the different degrees of a scale have each their own specific function. Nevertheless, the three main functions are those of the I, IV and V degrees, namely the tonic, subdominant and dominant. Now, take for instance the half cadence resulting from the perfect cadence, the IV-V-I progression (see article 7). It can be replaced by the II-V-I progression, which is very commonly used in jazz ─ since the II degree has the same sub-dominant function as the IV degree. This means you can interchange them easily within a given progression.
The table above summarizes the functions of each degree and indicates which chords share the same function and, thus, can eventually substitute one another.
Always bear in mind that you need to be cautious with substitutions. A substitution of the 1st degree with another tonic degree within the final perfect cadence of a song, for instance, will hardly ever work, since it will alter the nature of the cadence itself. In music, as in any other field, there is no absolute analogy!
In the next article we'll deal with substitutions linked to the dominant function.