Origin of the Names Given to Seventh Chords
Knowing the origin of the names given to the chords helps you remember their structure:
Origin of the Name Dominant Seventh The chord is built on the dominant degree of the major, harmonic and melodic minor scales. Major Seventh The triad and the seventh are major. Minor Seventh The triad and the seventh are minor. Diminished Seventh The triad and the seventh are diminished. Half Diminished Seventh The triad is diminished, but the 7th. is minor, so it is half diminished.
As shown on the table above, dominant sevenths are called by the name of the scale degree on which they are formed. The name given to major, minor, and diminished sevenths is determined by the type of triad and seventh interval on which they are built.