These next concepts will crucial to understand not only tonal harmony but also to understand how we can use this knowledge to escape the tonal pull exerted by certain chord types and instead work in an atonal or modal context. That said, a tritone is an augmented 4th or diminished 5th interval with three whole-steps. Considering that the octave has six steps, the tritone divides the octave in half.
In order to properly frame the importance of this interval let’s keep in mind the following:
- When intervals were discussed, I exemplified how intervals sound and the augmented 4th interval was one of the “hardest” sounding if not the most dissonant to your ears. The contrast between consonance and dissonance is what provides a sense of movement and direction to the music.
- The leading tone concept that I introduced when talking about scale degrees and how important it is to create a sense of resolution.
- In the same train of thought as the leading tone, the harmonized major scale has a diminished chord built over the 7th scale degree and this chord has a diminished 5th (a tritone) between the notes B and F. Besides being a chord with lots of tension, its resolution occurs when we go from Bm7(b5) to Cmaj:
As you can see, B and F respectively resolve to C and E, and each make a half-step movement towards their resting note
The relationship between tonic and dominant in conjunction with the dominant chord, which is the chord you get when thirds are stacked over the 5th degree of a major scale, is very important to reaffirm the strength of the tonic.
On top of that, in the context of the C major scale, a G7 chord has the leading tone B and the tritone interval between B and F in its formation. If you notice, the only note that sets apart the G7 from a Bo chord is the G. Every other note of the Bo chord is maintained in the G7 chord and thus they share similar properties:
As you can see, the tritone is a harmonic and melodic dissonance that is very important to tonal harmony because of the tension and instability that it creates. The tritone found in the dominant seventh chord helps to establish the tonality of a composition by pushing towards resolution and this is the case for all chords containing tritones that tend to “point somewhere”. Thus, and as we will see in other posts, it can also be used to avoid or delay the sense of tonality.
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