We can extend the idea of harmonizing the major and (harmonic) minor scales with triads to include four part harmonies. This means each note of the scale will produce a chord consisting of the triad plus a seventh.
In any Major key, we get the following 7th chords:
* The chord based on the 7th note of the major scale is only half diminished since it does not contain a double flat 7th. It only contains a minor 7th, but the rest of the chord is diminished (flat 3rd and flat 5th), so we end up with a minor 7 chord with a flat 5th.
In any (harmonic) Minor key, we get the following 7th chords:
The chords constructed from a minor key will have the same function as those in a major key, and can be used in similar contexts.
Still staying within each scale, we can then use substitutions such as minor 6th chords for minor 7th's and major 6th chords for major 7th's (see Chord Substitutions).
Notice there is only one dominant chord in each key. In both cases this is the fifth chord of the scale. This is what to look for in a chord progression to identify a key.
As before, we can add different 7th chords to the minor key by harmonizing the melodic minor and natural minor scales as well. Write these out as an exercise and harmonize them using four parts.