Triads are sort of the basic building block of homophony, which is a technical term for the dominant way that harmony has been used in Western music for the past few centuries. They are useful without doubt; you can play most any song with triads.
But remember this: triads are not the end of harmony as we know it; they are the beginning of harmony as we know it. Being dependent on triads today is sort of like driving around in a Model T. We can celebrate the history of triads but they have severe limitations in music and music development has grown past them.
I want to take a few minutes and discuss why this is true using the V chord which has the primary function of resolving down a fifth to the I chord. When we say a chord resolves, we are actually talking about how one or more notes in the chord resolves to a new note in the new chord.
Some of what goes on in a V – I resolution is just noise and is relatively unimportant. But there is one critical thing that happens. Here it is.
The third in a V chord is the 7th degree of the scale of the key and it wants to resolve to the tonic (1st degree). The 7th degree of a scale is not officially called a leading tone for nothing. It always leads to tonic. That is what is going on in a V – I progression that is important.
Now, a bit of history. Until roughly the time of Bach, this was the progression most often used. The music was polyphonic rather than homophonic but there were still chords being outlined in all those moving voices.
Some composers were starting to experiment with something radical though. They were adding the 7th to the V chord making it a V7. They had figured out that introducing a 7th into the V chord introduced another very satisfying resolution between notes when resolving to I. Here it is.
We still have the leading tone resolving to tonic, but now we also have the 7th in the V chord resolving down to the 3rd of the I chord.
If you sit down and play a V – I progression on the piano and then a V7 – I progression, you will not miss the improvement between the two. A V7 – I progression is better by far not only because that additional voice leading occurs but also because of something else that is introduced: a tritone interval being resolved.
The tritone is an augmented fourth and it occurs in every V7 between the 3rd and 7th. In this case, you see it between the F and B. The tritone interval is considered unsteady. It has to go somewhere. Think of it like holding two magnets end to end close together. You can hold them that way for a long time but eventually something is going to happen: either they are going to spring apart or they are going to come completely together.
The exact same thing happens with a tritone. The notes either resolve out by half step (as happens here) or they resolve in by half step. But there are few if any more satisfying resolutions in music as we know it than a tritone resolving.
On an interesting historical note, Bach was one the first major composers to use the V7, not because he discovered it but because the tritone was previously banned by the church which controlled almost all music. It was considered an evil interval. Finally, the church saw the error of its ways and today, the V7 – I resolution is the most recognizable chord progression in modern music.
That is just one example of why triads have limitations and why advanced harmony has moved on. Clinging to triads in 2013 in the face of more advanced harmony does not make much sense to me.
I wrote a few days ago about a little debate we were having on Facebook about whether I was incorrect to say V when I really mean V7 or V/iii when I really mean V7/iii. After reading this post, you may have an idea as to why I always assume that the 7th will be added. It does not change the function of the chord at all. It just improves the function and improves the resulting progression. From that perspective, I add the 7th automatically.
By the way, this all applies not only for the V7 but all the secondary dominants as well. The same thing happens in all of those progressions too.