A triad is three notes stacked in thirds. A major triad has a major third below and a minor third above (C–E–G); a minor triad is the other way round (C–E♭–G). They differ by one note, the middle one, by a half step.
The outer frame is identical in both: root to fifth is a perfect fifth, seven half steps. Everything bright or dark about the chord comes from the third in the middle — four half steps above the root makes it major, three makes it minor.
C major: C–E–G. Root, major third, perfect fifth.C minor: lower the E to E♭ and leave everything else alone.
Triad
Root to third
Third to fifth
Written
Major
Major 3rd (4 half steps)
Minor 3rd (3)
C
Minor
Minor 3rd (3 half steps)
Major 3rd (4)
Cm
Diminished
Minor 3rd
Minor 3rd
Cdim
Augmented
Major 3rd
Major 3rd
Caug
Every degree of a key carries a triad, and whether each one is major or minor is decided by the key itself rather than being something to memorize: in a major key I, IV and V are major, ii, iii and vi are minor, and vii is diminished. That is why chords get written as Roman numerals — capital for major, lowercase for minor.
Common questions
What is the difference between a major and a minor triad?
Only the middle note. Both have a perfect fifth from root to top, but a major triad puts the third four half steps above the root and a minor triad puts it three above.
How do you build a triad on any note?
Name the root, then stack the letters a third and a fifth above it, and check the half steps: 4 then 3 is major, 3 then 4 is minor.
What are the four types of triads?
Major, minor, diminished and augmented. They differ by the size of their two stacked thirds: major is 4+3 half steps, minor 3+4, diminished 3+3 and augmented 4+4.