We have already seen minor chords, e.g. A minor, which is the sixth degree of C. But Am has it's own scale as well, in fact there are several types of minor scales. We will talk about the harmonic minor scale of A:
The Am chord is built from the 1st, 3rd and 5th note of this scale: The main chords in the scale of Am are Am, Dm and E7: Like in the major key, they are the first, fourth and fifth degree of the minor scale.Notice that a minor key sounds sad compared to a major key.
A well known song in a minor key is 'Besame mucho': The transition chords also apply in the minor scale. They are the major transition chords.
To illustrate, there is a certain point in 'Besame mucho' where a transition chord is needed to go to the 4th degree (Dm). This transition chord is A7!
The transition chord to the 5th degree is also major. (That would be B7 in this example, we will get to that when the scale of E is talked about)