So far you've heard intervals that are stable (perfect) or colourful (major/minor). Now for the ones that sound restless - the clashing flavours that keep the palate interested. The tritone is the most unstable interval in music, and augmented/diminished describe what happens when you stretch or shrink any interval by a semitone.
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. It's like over-seasoning: a bit too much, creating a bright tension.
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
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.