Major vs Minor: One Note Apart
Major and minor are the first two words anyone meets in music, and they are usually explained as happy and sad. That explanation gets you moving and then gets in the way, because it makes the difference sound like a mood rather than a measurement. It is a measurement. It is one note, moved by the smallest distance there is.
The difference is the third
A basic chord is three notes: a root, which gives the chord its name, a note above it called the third, and a note above that called the fifth.
Move between a major chord and a minor chord built on the same root and the root does not change. The fifth does not change. Only the third moves, and it moves by one semitone — the smallest step in Western music, the distance from one key on a keyboard to the very next one, black or white. Down one semitone and a major chord becomes minor. Up one semitone and it turns back.
That is the entire structural story, and it is worth pausing on how small it is. Of the three notes, two are identical. The one that differs, differs by the least amount available. Everything people describe as the enormous emotional gap between these two chords is carried by that single step.
Why so little makes so much difference
Two answers, and they work together.
The physical one: the third is the note that decides the chord’s colour, and the ear is unusually sensitive to it. The root tells you which chord this is, the fifth adds weight and stability without adding character, and the third is left doing all of the describing. When the only expressive note in a chord changes, the chord changes completely, even though only a third of it moved.
The learned one: you have heard these two chords, in these roles, several times a day for your entire life. Western music has leaned on the contrast for centuries, so the distinction is heavily practised long before anyone names it for you. Part of what feels like a fact about sound is a fact about a lifetime of listening.
The emotional labels are a convention, not a law
Happy and sad are a useful first handle and a bad long-term description. Two things give it away.
The counterexamples are everywhere and are not obscure. An enormous amount of dance music is in minor and is not remotely sad — a great deal of club and electronic music, plenty of Balkan, klezmer and flamenco repertoire, and a long list of songs written to make people move. Meanwhile some of the most desolate songs ever recorded are in major throughout, and the desolation is in the words, the tempo and the voice. Tempo, rhythm, register, instrument, loudness and lyrics all do at least as much emotional work as chord quality does, and any of them can overrule it entirely.
And the association is not universal. It is a Western convention that took its current form over the last few centuries, and it is at least partly learned: listeners raised outside that tradition tend to show the effect far more weakly, when they show it at all. Before the convention settled, European music worked in a larger set of modes with no happy-versus-sad axis running through them.
A better pair of words is bright and dark, or open and inward. They describe the sound rather than prescribing a feeling, they survive the counterexamples, and they leave room for the thing composers actually do with these chords — which is not to announce a mood, but to shift the light on one and let you notice.
The same words apply to scales and keys
Major and minor also describe scales, and through scales they describe keys. The logic is the same one: the scale contains the third that the chord is built from, so a major scale contains a major third above its home note and a minor scale contains a minor third. A song in a minor key is one whose home note carries the minor version, and whose chords are drawn largely from that scale.
This is why the two words seem to be everywhere and to mean slightly different things each time. They do not. It is always the same measurement, applied to a chord, a scale or a whole piece.
Now go build them
Reading about a one-semitone difference is not the same as hearing it or having built one. The two lessons that go with this article do that work: one builds the major triad from any starting note and explains why it reads as bright and settled, the other shrinks the third by a semitone and shows what happens. Take them back to back and build both chords on the same root before you play them — side by side is where the difference stops being a description and starts being obvious.