In any given key certain chords are more common than others. For example, in the key of C the chords, C, F and G are usually present, and quite often the chords Am, Dm and Em occur. The reason for this is that each key has its own set of chords, which are constructed from notes of its own major scale. These chords are referred to as "scale tone" chords.
Consider the C major scale:
Chords are constructed by combining notes which are a third apart. For example, consider the formula for a major chord:
Using the C major scale, scale tone chords can be constructed by placing two third intervals above each note. This is illustrated in the following table:
Notice that the chords are named according to their root note (and hence use the root note’s scale). However, they are all C scale tone chords because they contain only notes of the C scale (i.e., no sharps or flats).
The method used for constructing scale tone chords in the key of C may be applied to any major scale. The result will always produce the following scale tone chords:
Thus in the key of G major, the scale tone chords will be:
and in the key of Eb major, the scale tone chords will be: