Chords Simplified by Using Thirds
Next, let’s begin to simplify the chord formulas using whole and half steps. 
The formula for constructing a Major chord would be: 
Two whole steps + one and one half whole steps 
Using Major and Minor Thirds to Build Chords 
Two whole steps is called a Major third. Any note combination that is two whole steps apart anywhere on the keyboard is called a Major third. 
One and one half whole steps is called a minor third. Any note combination that is one and one half whole steps apart anywhere on the keyboard is called a minor third. 
Now, using major and minor thirds, let’s construct our finished formulas: 
Minor chords (such as Am, Dm, and Em) 
minor third + major third 
Dominant seventh chords (such as C7, F7, and G7) 
major third + minor third + minor third 
Minor seventh chords (such as Am7, Dm7, and Em7) 
minor third + major third + minor third 
Major sixth chords (such as C6, F6, and G6) 
major third + minor third + 1 whole step (see note*) 
Major seventh chords (such as Cmaj7, Fmaj7, and Gmaj7) 
major third + minor third + major third 
*Note: one whole step is called a Major second. Any note combination that is one whole step apart anywhere on the keyboard is called a Major second. 
Record all of these chord formulas in your music note book. 
You can also note that: 
Major third = 4 half steps 
minor third = 3 half steps  
Major second = 2 half steps 
minor second = 1 half step