Exactly as the word implies, an Add Chord has a specified note (or notes) ADDED to the Triad chord.

-You can now form a Chord using up to 6 different notes.

-Since there Seven notes of a Major (or minor) scale, an add9 is the same as an add2 (after the 7th the notes begin from the 1st again).

FOR EXAMPLE:

From the C Major Scale (following W-W-H-W-W-W-H) Iis comprised of the notes: C-D-E-F-G-A-B.
The C Chord (Triad) contains the 1st-3rd-5th notes of the scale.  Namely: C-E-G.
The add2 is the addition of the 2nd note of the C Major Scale.  Namely the D note.

Therefore the Cadd2 uses the notes C-E-G-D

This Chord may be denoted C+2 Fig.1.  Example of an “add” Chord.

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The “Sus” Chord explained

-Simply put, to form the “Sus” Chord you either Lower or Raise the Second Note of the Chord.

-There are either Sus2 OR Sus4 Chords.

-Sus2 means to Lower the Second Note of the Chord by a semitone.

-Sus4 means to Raise the Second Note of the Chord by a semitone.

FOR EXAMPLE:
Csus2

From the C Major Scale (following W-W-H-W-W-W-H) Iis comprised of the notes: C-D-E-F-G-A-B.
The C Chord (Triad) contains the 1st-3rd-5th notes of the scale.  Namely: C-E-G.

The Sus2 is the lowering of the 2nd note of the C Major Chord from the E note to a D note.

Therefore the Csus2 uses the notes C-D-G

This Chord is denoted Csus2 Fig.2.  Example of a “sus” Chord. [wp-blogroll]