Tip for practice:
Try using seconds when you play major triads. A second will almost always work when a major triad is called for.
Okay, so what is a second?
A second is simply adding the second note in the chord. For instance here is a C2
LH / RH
C C / C D E G
Here is an F2
LH / RH
F F / F G A C
So if you are kinda "stuck" on triads and they are starting to sound bland to you, start sticking in second chords. They add a little flavor without adding too much contrast.
Now, you must distinguish a second from a ninth. A ninth has some type of seventh in the chord, whereas a second doesn't. So this would be a C9
LH / RH
C C / E G Bb D
The Bb note, which is a dominant 7th, gives the chord a sense of movement that doesn't exist with second chords. Seventh chords are very unstable, they want to move to resolve to another chord. Second chords are stable, which is why they can usually be used whenever you have a major triad.
Hope this helps someone . . .
Be Blessed!
