dominant 7th chord = major triad + minor 7th
- Example: G7 = G-B-D-F
minor 7th chord = minor triad + minor 7th (the
minor is referring to the triad)
- Example: Gm7 = G-Bb-D-F
major 7th chord = major triad + major 7th (the
major is referring to the 7th)
- Example: Gmaj7 = G-B-D-F#
So in a minor 7th chord the triad and the 7th are both minor, and in a major 7th chord the triad and the 7th are both major.
There are other types of 7th chord, such as the diminished and half-diminished 7th chord, but the above three are by far and away the most common ones in popular music.