Major Third

A C major chord is three notes: C, E and G. The C names the chord, the G adds weight and very little colour, and the E in the middle is doing the emotional work. Slide that E down one key to E flat and you are playing C minor, while the bottom note and the top note have not moved at all.

One key, and the chord goes from bright to dark. That is the entire difference between major and minor, which is why this is usually the first interval worth knowing by ear.

Counted in semitones a major third is four of them. Counted in letter names it is three, because music counts the note you start on as number one: C, D, E. That is where the word third comes from.

Play C to E, then D to F, then E to G. All three span three letter names, so all three are thirds, and they do not sound the same.

C to E covers four semitones. D to F covers three. The letters insist the two are the same kind of distance and your ears insist they are not, and the words major and minor exist to settle that argument. Four semitones is major, three is minor, and the letter names stay out of it.

The spelling matters as much as the counting. C to E flat is three semitones across three letters, so it is a minor third. C to D sharp sounds identical on a keyboard, covers only two letters, and is a different interval with a different job.

Every chord with a bright, settled base, which is most of what you meet:

The chords that leave it out are the minor family, which swaps it for a minor third, and the sus chords, which replace it with the note above or below. That replacement is the whole reason a sus chord sounds unresolved: with no third, the chord refuses to say whether it is happy or sad.

Measure from the first note of a scale to its third note and you find out which family it belongs to. Major, lydian and mixolydian all have a major third there, which is why all three sound bright however different they are further up.

Minor, dorian and phrygian have a minor third in that spot instead. It is the single most reliable thing to listen for when you are working out which scale a piece is using: everything else can wait until you have placed the third.

Play a low C and you are also hearing a stack of much quieter notes above it, in a fixed arithmetic order. Two notes sit together comfortably when those stacks overlap instead of colliding, and with a major third the overlap arrives early, which is what the ratio of five to four describes. More on that in the harmonic series.

Your piano does not play that pure ratio. Equal temperament makes every semitone the same size so all twelve keys stay usable, and the third pays for it by coming out about fourteen cents wide. Not enough to sound wrong, just enough to make a held major chord shimmer. Singers and string players are free to drift back to the pure version, and they do. See tuning and temperament.