Before we begin, I am assuming that you have a firm understanding of the musical alphabet and chords and such.
Every chord has a family made up of other chords. Here is the chord family of the C chord:
C Dm Em F G Am Bdim
As you might notice, the 1st, 4th, and 5th chords are major chords. This is true for every chord family. In addition, the 2nd, 3rd, and 6th chords are minor chords. This is also true for every chord family. Finally, the 7th chord is a diminished chord. This is also true for every chord family. This is how you make a chord family: Take the first chord and make it major, then take the 4th and 5th chords after that on the musical alphabet and make them major. Then, take the 2nd, 3rd, and 6th chords after the first chord on the musical alphabet and make them minor. Finally, take the seventh chord on the musical alphabet after the first chord and make it a diminished chord. These are the very basics, but there is more... much more mwhaha!
Names Of Chords In Chord Families
This section will help you understand what I am talking about when I say words like dominant and relative minor in the rest of the article. The dominant chord is the first, or 1 chord. The relative minor is the sixth chord that is minor, or the 6m chord. It is called the relative minor because it is the minor chord in the family that is closest to the dominant chord. There are also these numbers I keep using. The 1 chord is the first chord in the family and does not have anything after it because it is major, so we have the 4 and 5 chords. The 2m chord is called the 2m chord because it is the second chord in the family and it is also minor, so we have the 3m and 6m chords. Finally, the 7dim chord is called the 7dim chord because it is the seventh chord in the family and it is also a diminished chord. In addition to all of these there is something else: the secondary dominant.
Let me make a note at this point: The lines dividing chord families are very faint. You'll see what I mean in this next part.
The secondary dominant is also called the fifth of the fifth, or the 5/5 chord. This means that the secondary dominant comes from going to the 5 chord in the family, and then going up five more chords. For example: the 5/5 chord in the D chord family is E because you go to the 5 chord, A, and then count up five more on the musical alphabet to E. You may have noticed that E comes right after D. This is another way to find your secondary dominant: count one up from your dominant. Your secondary dominant can be a major chord or a seventh chord. In addition, you can go even farther and go to the fifth of the fifth of the fifth, or the 5/5/5 by counting up 5 more after the 5/5. You can keep doing this and go to the 5/5/5/5 and the 5/5/5/5/5, and so on.
I hope this has given you a nice intro to chord families. This is not supposed to be anything advanced, so I haven't explained everything that there is to know about chord families.