Much Western music uses chords which are constructed by stacked thirds, or tertian harmony.
Probably the most common chordal construction is the triad, which, as its name implies, is constructed by stacking three notes up from a root note. The major triad has a major third interval on the bottom and a minor third on the top
and the minor triad has a minor third on the bottom and a major third on the top. For more on intervals and how to recognise them, see Intervals.
If we stack another diatonic third on top of these triads we get a major seventh chord
and a minor seventh chord
Note: the major seventh has another major third added to the major triad while the minor seventh has a minor third added to the minor triad.
Other Types of Seventh Chords
Looking at a harmonised G major scale, we see that the majority of triads are either major or minor which will therefore become major seventh and minor seventh chords.
Before we create a diatonic seventh chord with the diminished triad, let us look at the triad found on the fifth degree, the Dominant, of the scale. When we harmonise a major scale with triads we get a major triad on the fifth degree. When we attempt to create a seventh chord on this degree, by adding another diatonic third to the triad, we get a slightly different construction from our major and minor seventh chords.
While a major seventh chord contains the stacked intervals of a major third, minor third and a major third and a minor seventh chord contains a minor third, major third and minor third, the seventh chord constructed on the Dominant contains a major third, minor third and another minor third.
This is called a Dominant seventh chord.
The diminished triad on the seventh degree, the Leading Note, of a major scale is constructed by stacking two minor thirds
To create a seventh chord we must stack another diatonic third. This gives us the intervals of a minor third, minor third and a major third.
This is called a Half diminished seventh chord or an F sharp minor seven flat five chord. For it to be a diminished seventh chord it would need to contain all minor thirds; a construction which does not occur diatonically in major keys.
The seventh chords described above would be found diatonically in any major or natural minor scale and also in the descending form of the melodic minor scale. See Types of Minor Scales.
Diatonic Seventh chords in Harmonic Minor
A harmonised harmonic minor scale produces the following diatonic triads:
Notice the two diminished triads on degrees ii and vii, the Supertonic and Leading note, and the two major triads on degrees V and VI, the Dominant and Submediant. There is also an augmented triad on the third degree, the Mediant. The construction of the augmented triad contains two stacked major third intervals.
By adding another diatonic third to these triads we get the following seventh chords
A harmonic minor scale harmonised in sevenths contains a half diminished seventh chord on the supertonic and a diminished seventh chord on the leading note. The major seven sharp five chord on the mediant results from the addition of a diatonic minor third to an augmented triad, and the minor major seventh on the root results from the addition of a major third to a minor triad. Interestingly, when a harmonic minor scale is harmonised as seventh chords all are different.
Diatonic Seventh chords in Melodic Minor
As noted above, the descending form of the melodic minor scale produces the same seventh chords as its relative major. The ascending form produces the following seventh chords.
Characteristically, the ascending harmonised melodic minor scale produces many of the same chords as in the harmonised harmonic minor scale, but there are two dominant seventh chords and two half diminished seventh chords in the melodic minor.
As with all theory, it is important to not only learn how these chords are constructed but also to learn how they sound. The sound of many of the seventh chords found in the harmonised minor scales are unique and it is worth taking the time to recognise their characteristics.