Examples of use
In major keys, Minor 7th chords works well on the II,III, and VI (2nd,3rd and 6th) degrees of the scale.
It can be used as a way of adding an extra color to a standard minor chord.
In D major, the Em7 is the chord on II and can form part of a II7
In C major, the Em7 is the chord on the III (3rd) degree of the scale.
This progression starts with the tonic C and moves to Em7 before moving down to another minor 7th chord, Dm7, forming a progression I-III7
In G major, the Em7 is the chord on the VI (6th) degree of the scale. Here's a common progression I-VI7
Minor 7th chords are one of the Diatonic 7th chords
we talk about in the theory lesson here