Major, minor, diminished and augmented

C-E-G and C-E♭-G differ by one half step in the middle note, and that single half step is the difference between a major and a minor chord.

Four ways to stack two thirds

A third can be major (four half steps) or minor (three half steps). Stack two of them and there are four combinations, which give the four triad qualities. A major third with a minor third on top makes a major triad: C to E is 4 half steps, E to G is 3. A minor third with a major third on top makes a minor triad: C to E♭ is 3, E♭ to G is 4. Both have a perfect fifth, 7 half steps, from root to top.

Two minor thirds make a diminished triad: B, D, F is 3 + 3 half steps, and the fifth from B to F is only 6, a diminished fifth. Two major thirds make an augmented triad: C, E, G♯ is 4 + 4, and its fifth is 8 half steps, an augmented fifth. Notice how the names work. Major and minor are named after their third; diminished and augmented are named after their fifth, which is a half step smaller or larger than perfect.

Changing one quality into another

Once you can spell a major triad, the others are one or two accidentals away. Lower the third by a half step and major becomes minor: C E G becomes C E♭ G. Lower the fifth as well and minor becomes diminished: C E♭ G♭. Raise the fifth of a major triad instead and it becomes augmented: C E G♯. The letters never change, only the accidentals, so A♭ minor is A♭, C♭, E♭ and not A♭, B, E♭.

How do they sound? Many teachers suggest listening for a character in each: major as bright or happy, minor as darker or sad, diminished as tense or unsettled, augmented as floating or dreamlike. These labels are learning aids, not rules, and composers use every quality for many moods. More useful for the ear is that diminished and augmented triads lack a perfect fifth, so they feel unstable and seem to want to move somewhere else.

Chord symbols follow the same logic: C for major, Cm for minor, C° (or Cdim) for diminished and C+ (or Caug) for augmented. To identify a chord on the page, stack it in thirds, name the root, then compare the third and fifth with the root's major key signature. If both match, the chord is major; a lowered third means minor; a lowered third and fifth mean diminished; a raised fifth means augmented.

The four qualities are not equally common. In a major key, major and minor triads are everywhere, the diminished triad appears once, on the leading tone, and the augmented triad does not appear at all among the seven triads built from the scale. It turns up in harmonic minor, on the third degree, and otherwise as an added colour from outside the key. That rarity is part of why an augmented chord catches the ear: it seldom grows out of the key itself.