The next figure shows the diatonic triad chords on the scale degrees of the C major key. We would have the same chord qualities, the same harmony annotations and different chord roots if we chose some other major key. This is the reason why we can examine only the C major key.
Figure: The diatonic triad chords in the C major key
In a major key, the diatonic triad chords have the following chord qualities:
Major triad on the first, fourth and fifth scale degree
Minor triad on the second, third and sixth scale degree
Diminished triad on the seventh scale degree
The next figure shows the diatonic seventh chords on the scale degrees of the C major key. We would have the same chord qualities, the same harmony annotations and different chord roots if we chose some other major key. This is the reason why we can examine only the C major key.
Figure: The diatonic seventh chords in the C major key
In a major key, the diatonic seventh chords have the following chord qualities:
Major seventh on the first and fourth scale degree
Dominant seventh on the fifth scale degree
Minor seventh on the second, third and sixth scale degree
Half-diminished seventh on the seventh scale degree