An add chord is a chord where an additional note, or extension, is added to a basic triad without changing the original chord structure or stacking extra thirds as in seventh chords. This added note gives the chord extra color or flavor without making it more complex like an extended chord (e.g., 7th or 9th chords).
In an add chord, we start with a major or minor triad (root, 3rd, and 5th) and add another specific note to it. The extra note, usually a 2nd (2), 4th (4), or 6th (6), added to the basic triad.
Add chords have a subtle, colorful quality that makes them feel rich and full, without the complexity of more extended chords.
There are several add chord shapes that are portable and can be played anywhere on the neck.
Portable add6 chord with the root notes on the 5th and 3rd strings
Portable add9 chord with the root note on the 4th strings