Interval Quality (Major, Minor, Perfect, Diminished, Augmented)
Going beyond the number to describe an interval's exact flavour: major, minor, perfect, diminished or augmented.
Explainer
Interval NUMBER (from Grade 1) tells you the letter-name distance. Interval QUALITY tells you its exact size in semitones.
2nds, 3rds, 6ths and 7ths can be major or minor. Unisons, 4ths, 5ths and octaves are normally perfect.
Make a major interval one semitone smaller and it becomes minor. Make a perfect or major interval one semitone bigger and it becomes augmented. Make a perfect or minor interval one semitone smaller and it becomes diminished.
Worked examples
C to E: Major 3rd
4 semitones — a major 3rd.
C to E♭: Minor 3rd
3 semitones — one semitone smaller than the major 3rd, so it's minor.
C to G: Perfect 5th
7 semitones — perfect intervals don't come in major/minor forms.
C to G♯: Augmented 5th
8 semitones — one semitone wider than perfect, so it's augmented.