In any key there are some chords which are more commonly used than others. The three most common chords are I, IV and V . These are the primary chords. With these three chords, it is possible to harmonize (accompany with chords) any melody in the key, because between them they contain every note of the scale, as shown here in the key of C.
This principle applies to all keys. Here is the Eb major scale harmonized with chords I, IV and V .
The following table shows the primary chords in all twelve major keys.