Definition
SUS is the abbreviation for suspended. A suspended chord is neither major nor minor since the suspension refers to the third. The third characterizes the chord as major or minor, so when the third is not there we have a suspended chord. In this way, a Power chord that has only tonic and fifth is a suspended chord, since it does not have the third. In the case of the Power chords, they do not take the SUS symbol in the chord symbol, instead, their chord symbol receives the number 5. Thus, the C Power chord symbol is C5. The suspended chords instead receive the SUS word (from suspended) in the chord symbol and this indicates that besides suspending, the third is replaced by another note that can be the fourth (Sus 4) or the second (Sus 2).
Add 4 chord
The sus4 chords (Csus4) should not be confused with the chords added fourth (C4 or Cadd4), because in this case we just add the fourth without removing the third. This chord is very dissonant because the third conflicts with the fourth due to the separation of a semitone between them.
Let’ build, for example, The ASus4 chord. The major chord A is formed by the notes A which is the tonic, C#, the major third, and E, the perfect fifth. Replacing the C# by D is the fourth in the A scale we then have A, D, and E.
Another rule
If you don’t know which notes in the scale are sharpened or flatted you can use this other rule. Just count the halftones between the notes in the chord. So, to build a SUS4 chord count 5 halftones between the tonic and the fourth and then another tone between the fourth and the fifth.
