There seems to be a lot of confusion about when to use suspend vs. add (e.g. sus2 vs. add2) and when to use single or compound interval numbers (e.g. 2, 4, 6 or 9, 11, 13). Let me try to shed some light on this, especially for non-guitarists writing guitar parts.

Interval Naming & Chord Tensions
This one is pretty simple. If the chord is not a 7th chord, you always use the 1st octave interval names (2, 4, 6). If the chord is a 7th chord then you use the compound interval name (9, 11, 13).

So, if you write a C9: any guitarist worth their salt will play:

What is a suspended chord anyway?
A suspended chord, by definition, is a chord in which you replace the 3rd degree of the chord with the named interval. So a sus4 chord is a chord in which you replace the 3rd degree in the chord with the 4th degree. Likewise, a sus2 chord is a chord in which you replace the 3rd degree in the chord with the 2nd degree.

So, a D(sus4) is asking the guitarist to take a D major chord and replace the F# with a G (the 4th in the D scale) to make this chord:

Similarly writing D(sus2) would be asking the guitarist to take a D major chord and replace the F# with an E (the 2 in the D scale) to make this chord:

Because the sus4 chord is the most popular suspend chord, simply writing D(sus) would automatically imply a D(sus4) chord. You can leave the 4 off of a (sus4) chord designation. It is assumed to be 4 if no number is given.

Is it (add2) or (add9)?
Now, I often see this mistake in written music: there will be a C(sus2) written where it was really intended for the guitarist to play a C(add2).  So, you may ask, what’s the difference?

As noted above, a suspended chord has no 3rd. You replace the 3rd degree with the suspended interval. An (add) chord simply adds an extra note to the chord without replacing any of the notes from the triad.

So, a Csus2 would be:

and a C(add2) would be:

Note, the suspended chord is still a triad (3 note chord: C, G, D, G – low to high. Note: there are 2 Gs in the chord. They only count as 1 note), while the (add2) chord is a 4 note chord (C, E, G, D – low to high).

And simply, it is always (add2) and never (add9). An (add9) chord would be indicating the presence of a 7th in the chord so a C(add9) is simply a C9:

See the difference?

So, if you come across an (add9) chord, just know it was meant to be an (add2) and go with it. If you are writing for guitar, please refrain from (add9) chords and use (add2). If you really want a Dominant9, just write C9 (or E9, or whatever).