Major and minor intervals

Seconds, thirds, sixths and sevenths come in major and minor. Minor is always exactly one semitone smaller than major.

Fourths, fifths, octaves and unisons never do — they are perfect, and altering them makes them augmented or diminished instead.

The four pairs

The third does the heavy lifting

Of the four, the third matters most, because it is the interval that decides whether a chord is major or minor.

Play C–E and it sounds open and bright. Play C–E♭ and it darkens immediately. Nothing else changed. That single semitone is the reason "major sounds happy, minor sounds sad" is such a durable cliché — it is an oversimplification, but it is pointing at something real.

Inversion flips the quality

Turn an interval upside down — move the lower note up an octave — and two things happen predictably:

• The numbers add up to 9. A third inverts to a sixth, a second to a seventh, a fourth to a fifth.
• Major becomes minor and minor becomes major. Perfect stays perfect.

So C up to E is a major third; E up to C is a minor sixth. This is genuinely useful: learn thirds well and you get sixths free.

Reference points for the ear

The standard trick is to attach each interval to the opening of a tune you already know. Use songs you actually know rather than the canonical list, or it will not stick.

Descending is a separate skill

Recognising a major sixth going up does not mean you will recognise one coming down. They are stored differently. Practise both directions deliberately, or you will end up reliably good at half the job.