Along with the dominant seventh, the minor seventh chord is among the most frquently occuring sounds in contemporary music. Its sound is less dark than that of a pure minor triad. Contrary to major seventh chords, which are notated as M7, minor sevenths are usually encountered as m7 or -7. The minus sign refers to the third and seventh, which have both been decreased by a half step when compared to the major seventh.
Building the Chord
The difference between the major and dominant is that the seventh is a half step larger in the major ( C-B vs. C-Bb. In the minor, not only is there a minor seventh, there is also a minor third ( C-E vs. C-Eb). So, it is apparent that in the progression of major, dominant and minor, the intervals within the chord become smaller.
Consider A-7. This minor seventh is composed of the notes: A C E G. By breaking the intervals down, it can be seen that between A and C there is a minor third. This third gives the chord its minor tonality. Between C and E is a major third ( note that there is still a perfect fifth between A and E). Finally, between E and G is another minor third. Another way to see this is that between A and G is a minor seventh.
Minor Seventh Chords within the Diatonic Scale
Minor chords occur both in the major and minor diatonic scales. Within the major scale there are three naturally occuring minor chords. These exist in positions I, III, and VI. Take the C major scale. On the second step is D minor, built of the notes: D F A C. Again, the chord is composed of a minor third, major third and another minor third stacked upon each other. The same goes for E (III), composed of E G B D and A (VI), A C E G.
In the minor mode the minor chords occur in different positions, although they built the same. Minor chords occur in positions IV and V (see below for more on the V chord in the minor mode). In the key of A minor (no sharps or flats), D and E are the two positions where minor chords are found. It is a good idea to play these chords on a piano to truly see the intervals that make up the chord.
Raising the Third and Creating a Leading Tone
In the case of the V chord in minor modes, although it is technically minor, most commonly that chord is altered to produce a different effect. To use Am as an example, again E is the V and it would technically be played as E G B D, but since it's the dominant in that key, theconvention is to raise the third, or the leading tone. Doing this produces a stronger pull back to the Am, which is the function of V chords.
Playing the E chord as E G# B D instead of E G B D produces a much stronger and convincing resolution to Am. This is because the raised third in the E chord (G#) is only a half-step away from A and its natural tendency is to move up another half-step and resolve on the tonic. However, this isn't to say that V chords in minor can't be minor themselves. It all depends on the desired effect and mood of the music.
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