Major Chord Rule

Take any major scale, say C Major. Pick the 1st, 3rd and 5th notes from the scale. For example in the case of C Major scale - C, E, G

These three notes, played together would constitute the C Major chord.

Taking D Major scale, we get - D, F#, A as the three notes that would make up the D Major scale.

Proceeding this way, we would have 12 major chords from A Major to G Major.