Tonics can be either major or minor, but a dominant chord is always major or suspended. In the example below this next one, the tonic would be CMaj7 and the dominant would be G7. Dmin7 could be considered a G9sus4, which puts it into the dominant category. A 7(sus4) chord is a chord with a suspended 3rd. There is no third but a 4th substituted for the third. The G7sus4 chord would look and sound like this.

 


A seasoned jazz improviser will have many different methods of changing the sound of either of these groups, especially with the dominant group. Some harmonies are somewhat enigmatic and can be considered to belong to either group. As an improviser, what you choose to play is the deciding factor. Please play the following chord progression. (Drop–2 chords.)
 
The CMaj7 is a tonic harmony. A major 7 chord is composed of a major triad plus a major seventh. In this case, the major seventh interval is the note B. B is one half-step below C

The G7 is a dominant 7 chord. It sounds like it wants to move to CMaj7. A dominant 7 chord is one in which there is a flatted 7th—a whole step below the root, rather than a half step as in a major 7 chord—added to a major triad. For G7 the minor or b7 note is F. The note F is one whole step or two frets below G.

You will notice that the Dmin7 and G7 in Example 1.3 have the same outer voices—the outer voices are the top and bottom notes of the chord voicing. The Dmin7 chord in this instance is one of the harmonies that could go either way—tonic or dominant. It is very similar to a G7sus chord or an unresolved dominant sound. A minor 7 chord is a minor triad plus a minor 7th (one whole-step below the root). You will note that in the Dmin7 chord the minor 7th note, C, is one whole step (two frets) below D, the root of this chord.