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6.18.5 How to Write Chord Progressions, Rule #5: Take Advantage of Dominant Chord Instability
The dominant seventh chord is inherently unstable (all dominant-seventh type chords contain the tritone; minor sevenths do not) and can therefore serve as a transition chord to another chord. The dominant seventh is probably the most useful and versatile of all chords.
Any chord can always progress to any dominant seventh chord without sounding unpalatable.
But watch out when you go the other way. Moving from a dominant seventh to its own major or minor triad does not sound palatable. For example, try to avoid doing this:
G7 – G
or
G7 – Gm
or at least have a good reason for doing it.