How to Use Music Theory to Build Major Triads
Because they’re made of intervals, music theory indicates that triads are affected by interval quality. The quantities of the notes that make up the triad are intervals of root, third, and fifth, but it’s the interval quality of each note that changes the voicing of the triad.
A major triad contains a root, a major third above the root, and a perfect fifth above the root. You can build major triads in two ways.
Half‐step counting method
You can count out the half steps between notes to build a major triad using this formula:
Root position + 4 half steps + 3 half steps (or 7 half steps above root)
Check out C major on the piano keyboard. The pattern stays the same no matter the root, but it looks trickier when you move away from C. Notice the pattern of half steps between the root, the third, and the fifth.
First, major third, and fifth method
The second way to construct major triads is to simply take the first, third, and fifth notes from a major scale.
For example, if someone asks you to write down an F major chord, you first write out the key signature for F major.
Then you write your triad on the staff, using F as your root.
If you were to build an A flat major chord, you would first write down the key signature for A flat major and then build the triad.