## Tip #154 – Cheat-Sheet for Four-Part Fourth Structures

Okay, the title may seem really confusing, but this is what it is:

This post is about how to create fourth structures (harmonies built on intervals of fourths) with four notes.

To read the cheat-sheet below, take a melody note and decide what function is it in relation to the chord. Then, you add three remaining chord member notes below it. Finally, you analyze the chord in relation to the melody note being the tonic:

So, say that I have the note A, and I want it to be the m3 of a chord:

Because A is the m3 of the chord, that means the notes below it (from highest to lowest) are E as the b7, B as the 11, and F# as the root 1. We can thus analyze this as vi 11 in the key of A major:

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## Tip #151 – Using Fourth Structures in Harmonization

Continuing with a topic discussed the other day, we are going to take a look at how we can harmonize a melody using these fourth structures.

First, start off with a melody:

Then, add the diatonic fourth below the melody (this can either be a P4 or a tritone):

Now, we are going to do a closed fourth voicing, which means that we will add a third below the melody note. However, for the harmony that has a tritone, we are going to add a third ABOVE the bottom note:

If we instead wanted to do open fourths, we would need to stack two intervals of a fourth on top of each other. So, that simply means adding another fourth below the fourth from the previous example:

But, notice how the one with X’s have a tritone in them. Technically, that does not exactly fit the definition of an open fourth structure. To edit this, we use chromatics to adjust the harmonic intervals:

And there you go!

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## Tip #150 – Difference Between Open and Closed Fourth Structures

Just like triads (tri- three) that are made of thirds, you can construct chords out of the perfect fourth interval.

Simply, and open fourth structure is a chord built from two P4 intervals on top of each other, spanning a m7 interval. A closed fourth structure is one that spans an interval of a P4 with a third below the melody note (either m3 or M3 depending on the diatonic scale)

However, in the case that the interval of the diatonic closed fourth chord is a tritone, the best way to reharmonize it is to create an interval of a third between the bottom note instead of the top:

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