As the third and seventh can be major or minor and the fifth can be diminished or augmented there are 10 different possibilities or varieties of seventh chords. The following table outlines these 10 varieties and is taken from the section on chord spelling.

The seventh note can be either a whole tone or semitone below the octave (diminished seventh chords have the seventh note three semitones below the octave). A major seventh chord has the seventh one semitone below the octave. A minor seventh note is flattened or lowered by another semitone making it a whole tone below the octave. A major chord with a flattened seventh note is called a dominant seventh chord.

Dominant/minor sevenths occur more commonly than major sevenths. Because of this a '7th' is taken to be a flattened seventh unless otherwise stated. This means don't play a major seventh unless the chord specifically says so, i.e. Cmaj7. Dominant seventh chords are often used in chord progressions where the following chord is a fourth higher. e.g. An E7 chord leads smoothly up to an A7. Seventh chords are very common in Blues and Jazz.

If you refer to the section on modes you will notice that only Ionian and Lydian have a major 7th and that all the rest have a flattened 7th.

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