So far you've heard intervals that are stable (perfect) or colourful (major/minor). Now for the ones that sound restless because of their close or uneven interval relationships. The tritone often creates strong tension in tonal harmony,
The tritone - 6 semitones
Exactly halfway through the octave. 6 semitones up or 6 semitones down - either way you land on the same spot. This symmetry makes it sound unresolved and tense, like a question with no answer.
C to F# - 6 semitones, the exact midpoint of the octave
The "devil's interval": Medieval church musicians called it diabolus in musica. It was considered so dissonant it was avoided entirely. Today it's essential in blues, jazz, metal, and horror soundtracks.
Where the tritone lives
The tritone is hiding inside every dominant 7th chord. In G7 (G-B-D-F), the B and F are a tritone apart. That built-in tension is why dominant chords want to resolve so badly.
When the tritone resolves (B goes up to C, F goes down to E), you get a C major chord. Tension to release - that's what makes music move.
Augmented - one semitone wider
Take any perfect or major interval and widen it by one semitone - you get an augmented interval. The wider interval changes its pitch content and can create extra tension, especially when its notes sit close to tones in the surrounding harmony.
Augmented examples
Diminished - one semitone narrower
Take any perfect or minor interval and shrink it by one semitone - you get a diminished interval. It sounds compressed and dark.
Diminished examples
Notice that the augmented 4th and diminished 5th are the same pitch (6 semitones). They're enharmonic names for the same interval - the tritone. Which name you use depends on the musical context.
The full quality spectrum
How qualities relate (using 5ths as an example)
Each step = +1 semitone
Using 3rds as an example
Try it
Play B and F together, then resolve B up to C and F down to E. Before hearing the resolution, predict which direction each note will move. Repeat with G7 resolving to C major and listen for the same tritone motion inside the chord.
Finally, spell the same six-semitone distance once as an augmented 4th and once as a diminished 5th. The sound is the same; the musical spelling depends on context.
Key takeaway
The tritone (6 semitones) is the most unstable interval - it creates tension that wants to resolve. Augmented = one semitone wider than perfect/major. Diminished = one semitone narrower than perfect/minor. The aug 4th and dim 5th are the same pitch (both 6 semitones) - enharmonic equivalents.
Next: what happens when intervals stretch beyond an octave - compound intervals like 9ths and 13ths.
Hear tension and resolution
Generate a dominant 7th chord in Starts and listen for the tritone tension inside it.