Recognising a chord's quality, and the note that decides
Hearing whether a chord is major, minor, diminished or augmented without counting notes; the third is the only note that changes between major and minor, and the difference is enormous.
A chord sounds, and before any arithmetic the ear already knows whether it is open or closed, settled or unstable. That is recognising the quality: major, minor, diminished, augmented, and in seventh chords the five combinations with the seventh. You do not count note by note; you learn each one's colour, the way you learn the colour of an interval.
Start with major and minor
Between the major triad and the minor one, a single note changes: the third, a semitone down. The difference in sound, on the other hand, is the largest harmony has.
Play the two in sequence, many times, until the difference is obvious. Then change the root and do it again: the colour of "major" and of "minor" does not depend on the note, and it is the colour that is learned. Whoever tells the two apart reliably already has the most important thing.
Then the two odd ones
The diminished triad shrinks the fifth and is left without a floor; the augmented stretches the fifth and is left in the air. Both sound "wrong" in opposite ways, which makes them easy to separate from major and minor, and easy to separate from each other.
The diminished asks to resolve downward and inward; the augmented asks for nothing and stays suspended. Play each followed by a major chord and hear the difference in direction.
Seventh chords
With the seventh, quality has five common colours, and what you hear first is the pair third-and-seventh:
- Major seventh — : still, luminous.
- Dominant — : the major third with the minor seventh, the chord that pulls.
- Minor seventh — : closed and stable.
- Half-diminished — : minor with the fifth shrunk, the chord of preparation.
- Diminished seventh — : everything shrunk, everything asking to move.
The order for learning is the same as for triads: first separate the dominant from the major seventh, which differ only in the seventh and sound completely different; then the minor seventh; then the two diminished ones.
What gets recognised first in practice
Listening to a song, the trained ear recognises the quality and the bass before anything else, and the extensions afterwards, if at all. A ninth chord and a seventh chord have the same quality and the same function; knowing it is "minor seventh" is already enough to accompany, and the ninth is a detail confirmed at the instrument.
How to train it
This site's practice plays a triad, or a seventh chord, and asks for the quality, with the options that get confused with one another. Start with the triad practice and only move to seventh chords when major and minor never go wrong. As with intervals, a little every day and spaced repetition do the work; the memory of a colour is slow to form and, once formed, does not fade.
What to take from here
A chord's quality is a colour you learn, not a sum you do. Major and minor differ in the third and sound opposite; diminished and augmented go wrong in contrary directions; in seventh chords, the third-and-seventh pair is what you hear first. In practice, quality and bass come before everything, and function, which is another skill, comes after.