It's easy to work out the primary chords in any key – they are simply the chords (or triads) built on notes 1 (I), 4 (IV) and 5 (V) of the scale. (Note how Roman numerals (I, IV and V) are used to indicate the chords). So, for example, in the key of C major the primary chords are C (I), F (IV) and G (V).
