For this music lesson we're going to assume that you already know what a triad is and how to build one. Complex sounding chords, and often just as difficult to finger, involve more than the basic triad which makes up a chord. 7th chords are reasonably common in most forms of popular music. But, just how do you make one?
This lesson will deal with the Dominant 7th chord, which is usually just called the 7th chord and the minor 7th chord. There is also a Major 7th chord. There's a cheat way of knowing how to create a Maj7th chord. Simply add the note to your Major chord which is a semi-tone lower than your octave. For example, for an A Major chord add an A flat note, for E Major add an Eb, for C add a B. The true theory is to add a 3rd note above the 5th of your triad. Let's look at building a 7th chord and a minor 7th chord in the key of C. Afterwards see if you work out some Major 7th chords for yourself.
Ok. the 7th (or Dominant 7th chord). Add a minor 3rd above the 5th. The triad for a C Major chord is C (1st), E (3rd), 5th (G). The minor 3rd note above G is a B flat so your C7th chord is C-E-G-Bb. Check the diagram, and play through the variations - arpeggio, chord, run. The runs in both diagrams play through to the octave and back again.
To create a minor 7th chord we follow the same rule. The easiest way to think about it is to create your minor chord first by using the flat 3rd and then add then minor 3rd above the 5th as we did earlier.
Possibly by now if you are a guitarist you are wondering why the key of C. It's an easy chord for piano. If you're finding the stretches a bit difficult move everything up a tone. Now you are in the key of D. Between your keyboard and guitar you might find that d minor 7 is a comfortable key to play in.
That's the theory of constructing your 7th and minor 7th chords. It's not that difficult if you know your scales and get into a little practice of this. But... the cheat version actually works the same for these too. Add the note that is a full tone below the octave to either your Major or minor chord. Have fun!