To create a "minor seven" you have to add a seventh to a minor chord. In a Cm7 for example this would be a Bb ...and not B which would be a major 7th (maj7) and comes very rarely with a minor chord.
Instead of going up seven steps in the minor scale you can count back a whole step (or two half steps): C → B → Bb
- An m behind the root tone represents "minor" plus a 7 for the 7th, e.g. Em7, Bbm7, etc.
Instead of the m you'll often find a minus sign, e.g. E-7, Bb-7, etc.
Em → Em7
To change an E minor chord (Em) into an E minor seven (Em7) chord we look for a root tone E and lower it by two frets to get the seventh D.
Each fret on the guitars fretboard represents a half step
Drag the slider under the chord shape to the right to change the Em into an Em7 with this method. Also check out the whole thing with displayed intervals and tones and watch carefully how the chord is changing.
A different way to get a seventh is to raise a fifth (5) by three frets (half steps).
In this example we provide an Em chord with even two sevenths with our two methods - sounds good!
If our chord shape contains only one fifth we can raise it nevertheless. It's no problem to omit the fifth in a seventh chord (not for chords with b5 or #5). Root, third and seventh are essential for the character of a seventh chord.
Am → Am7
Same procedure with Am. It's possible to lower the A on G string by two frets. The open G string is the seventh of A.
...and here we additionally raise the fifth by three frets (half steps) again.
Dm → Dm7
That way a Dm changes into a Dm7.
To generate a seventh by raising the fifth makes not much sense here.
Dm7 as barre chord
I'd recommend to play the Dm7 as barre chord. At the fifth (V.) fret you'll find a shape equivalent to the Am7 we talked about before.
Here you can again (like before with Em7 and Am7) replace the fifth on the high E-string by the seventh located three frets higher.
Move that chord shape along the guitars fretboard to obtain other minor 7th chords, like e.g. Cm7 (two frets lower), Ebm7 (one fret higher), etc.
Gm → Gm7
You can treat the Gm barre chord exactly like the Em discussed in the beginning - just three frets higher at the III.fret.
Here as an example the Gm7 chord including the seventh two times. That sounds great and is handy to play.
Further chord shapes
There are quite a few different chord shapes for minor 7th chords. The root does not always have to be the lowest note, for instance if you've got a second instrument (bass, piano,...) playing the bass note, or if you're packing more than one chord shape of the same chord into a bar to make your accompaniment more interesting.
This lesson is not intended for showing all possibilities. The construction of minor seventh chords should be clear by now. The chord finder shows more m7 chord shapes - you can also fiddle out others by yourself with the help of the analyzer function!