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The problem it solves
Sometimes an interval is neither perfect nor major nor minor: it has been widened or narrowed by a semitone. You need to name it correctly without changing its number and to recognise the tension this adds.
Detailed theory
Key idea
Altering by a semitone keeps the number (same note names) and only changes the quality: wider = augmented, narrower = diminished.
From a perfect: perfect + semitone = augmented, perfect − semitone = diminished. From major/minor: major + semitone = augmented, minor − semitone = diminished.
Understand it
So far you have seen perfect intervals and major or minor ones. But an interval can be altered: if you move one of the notes by a chromatic semitone (adding a sharp or a flat), you change its quality without changing its number, because both notes keep their letter name.
If you widen it by a semitone, the interval becomes augmented; if you narrow it by a semitone, it becomes diminished. From a perfect interval the path is direct: perfect plus a semitone is augmented, perfect minus a semitone is diminished (perfect intervals never pass through major or minor). From a major or minor one, instead, the chain is: major + semitone = augmented, and minor − semitone = diminished.
Look at it with the fifth. C–G is a perfect fifth (7 semitones). If you raise the G to G#, you get C–G#, an augmented fifth (8 semitones). If you lower it to Gb, you get C–Gb, a diminished fifth (6 semitones). All three are still fifths, because G, G# and Gb all keep the note's name: you have only changed the exact size.
The most famous case is the tritone: the augmented fourth and the diminished fifth are both six semitones and split the octave exactly in half. C–F# is an augmented fourth; B–F, a diminished fifth. Historically it was called the diabolus in musica for its very tense sonority, and even today it asks to resolve toward a more stable interval.
An analogy: the interval number is like the lane of a motorway and the quality like the speed. Augmenting or diminishing does not change your lane (you are still in a fifth), it only speeds up or slows down the size a touch. That is why C–G# remains a fifth and does not suddenly become a sixth.
Staff & keyboard
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The tritone F–B: an augmented fourth of six semitones that splits the octave in half. Press the keys to feel the tension asking to resolve.
How to recognise it
How it's written
They are marked with "aug" or the + symbol for the augmented one, and "dim" or the ° circle for the diminished one: A5 (C–G#), d5 (C–Gb), A4 (C–F#). On the staff the number stays the same through the degrees it spans; what changes the quality is the accidental (a sharp or a flat) added to one of the notes.
How it feels
Augmented and diminished intervals sound unstable, with a push or strangeness that perfect and major intervals lack. The tritone (C–F# or B–F) is the clear example: it sounds tense, suspended, as if it asked to move toward a note of rest. Play C–G (stable) and then C–Gb (uneasy) to feel the contrast.
Common mistake
Changing the number when you alter the interval: C–G# is an augmented fifth, NOT a sixth, because G and G# share the note name.
Confusing the diminished fifth (C–Gb, 6 st) with the perfect fourth (C–F, 5 st) just by sound: you must look at the note names to fix the number.
Try it
Play C–G (P5), then C–G# (augmented 5th) and C–Gb (diminished 5th): the same fifth in three sizes.
Play the tritone C–F# (augmented 4th) and try resolving it by opening it toward B–G or closing it toward D–F: you will feel the tension seek rest.
On the instrument
Interval distance
C–G: the reference perfect fifth, seven semitones. A stable starting point before altering it.
Interval distance
C–G#: the augmented fifth, eight semitones. One semitone wider than the perfect one; it is still a fifth because G# keeps the name.
Interval distance
C–Gb: the diminished fifth, six semitones. One semitone narrower than the perfect one: it is the tritone, tense and wanting to resolve.
Where it's used
- Recognising the tritone
- Identifying the augmented fourth and the diminished fifth that create the tension asking to resolve.
- Building aug and dim chords
- Using augmented and diminished fifths to form augmented and diminished chords.
- Naming chromaticism well
- Keeping the interval number (fifth, fourth…) even when a note carries a sharp or a flat.
Examples
Interval distance
C–F#: the augmented fourth, six semitones. The other face of the tritone, just as tense as the diminished fifth.
Interval distance
B–F: the diminished fifth within C major, the tritone that tends to resolve toward C–E.
Exercises
Feel the tritone and altered intervals
Aural and visual recognition of intervals.
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Mini test
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0/8 answeredQuestion 1/8
What happens when you alter an interval by a chromatic semitone?