"Try Some Tritones"

Intervals within a single octave
(expressed as difference in semitones)
Minor / Diminished Major / Perfect
1 Min 2nd 2 Maj 2nd
3 Min 3rd 4 Maj 3rd
5 Perfect 4th
6 Diminished 5th 7 Perfect 5th
8 Min 6th 9 Maj 6th
10 Min 7th 11 Maj 7th
12 Octave

Intervals

The table above displays the 12 intervals that exist within a single octave. These intervals express the characteristic harmonic resonance that occurs between two tones at a given distance. The more that the two tones compliment one another the more consonant they are said to be. The most consonant interval is the Perfect Fifth. (In the Major Scale a Perfect 5th attends every tone except VII) The more two notes interfere the more dissonant we consider them.

Harmonies and Leading

The dynamics of musical leading depend on the interplay of consonance and dissonance. When tones are consonant - lacking harmonic tension - they tend to leave more freedom of movement. Conversely, dissonant harmonies set up more explicit movements.

Tritones

Of the twelve intervals described above only one is its own reciprocal: the Diminished 5th, also called the Augmented 4th. When two notes a Diminished 5th apart are rung together it sets up a harmonic tension which is called a tritone. "If there is a bright center to the Universe then this is the harmony it is farthest from."

In the Circle of Fifths notes that are closer together are more consonant and notes that are farther apart are more dissonant. The Diminished 5th interval extends to exact opposite sides of the Circle of Fifths. And it also extends diametrically across the Circle of Semitones, an arrangement in which tones near to one another are similar in tone rather than harmony.

The tritone is the place where everything culminates, where there is no place left to go but back. But where is the tritone leading us back to?

A Tritone Example

The most familiar tritone shows up in the V7 chord. In the key of C this is G7. Playing the G7 chord sets up a tritone which wants to immediately resolve to C, which is down a 5th from G. In fact, the movement to the 5th below is the strongest leading tendency of the tritone interval.

The G Dominant 7 chord has the most power to lead because it contains not just the tritone but three other tones with tendencies to resolve to C. So altogether there are four harmonic components that conjoin together to insist on a resolution to C Major:

The Leading Note (B)
The 7th note of the C major scale, the leading note B, tends to resolve naturally to C (the tonic). B is the 3rd of G7, and the movement from B to C can be heard in the V-I resolution, when the 3rd of the dominant chord of G major (B) resolves to the root note of the tonic, C major.
Fourth Note (F)
The 4th (F) has a strong tendency to resolve harmonically. When included as a 7th in the dominant chord, G7, it moves to E, the 3rd of the scale and the tonic chord. Try playing C major with F (Csus) and move F down to E to hear how this movement works with the note C.
Fifth Note (G)
The movement B to C can be heard using a two note V chord. With G this resolves C major. The bass (G) moves naturally up a fourth (down a 5th) - G to C.
Tritone
The combination of F and B sets up a strong tension that needs to resolve. The tritone interval is the same when F is placed above B. Each note must still move by a semitone for resolution.

Every Tritone Is Really Two Tritones

Depending on how you play the notes of a tritone it can have two different ways of resolving. F below B is an Augmented 4th tritone, the kind that likes to resolve to C. F above B has a tritone interval of a Diminished 5th, and this kind prefers to resolve to Gb/F#. In this case the two tones simply resolve in opposite directions. Instead of F-E and B-C the movement is F-F# and B-Bb.

The resolution to Gb/F# can be used with F below B, and likewise the movement to C can be used with F over B. It all depends on the context - and your ear.

Some material is reproduced here from The Complete Guitarist by Richard Chapman.