The chords of a key · Music theory
The triads on each degree in major and minor, tonal functions and cadences.
A triad on each degree
On each degree of a scale a triad is built from the notes of the scale itself, and named by its degree in Roman numerals. In major, I, IV and V are major; II, III and VI, minor, and VII, diminished.
In C major they are C major, D minor, E minor, F major, G major, A minor and B diminished. It is the same in every major key.
The pattern repeats in every major key: major, minor, minor, major, major, minor, diminished. Once you know it by heart, the scale alone gives you all its chords.
Primary and secondary degrees
I, IV and V are called primary because together they contain all seven notes of the scale: in C major, C–E–G, F–A–C and G–B–D. With them alone you can harmonize almost any simple melody.
II, III and VI are secondary: they bring a minor color to the major key and give variety. VII, diminished, nearly always behaves like a V without its root.