bookas58 provided part of the answer: the 9th scale degree is the 2nd an octave higher. Thanks!

But there's also a slight difference between a sus (suspended) chord & add chord.

A "sus" chord replaces ("suspends") the 3rd with a 2nd, while an "add" chord adds the 2nd (or 9th) in addition to the 3rd. See theory below!

Originally Posted by: cj.stevens

Can you explain the theory behind that statement, and what is meant by other musical contexts?

Chords are named by the scale degrees they consists of. Major chords consist of the major scale degress 1st, major 3rd, 5th. So those three notes are assumed in all major chords.

An A major is the 1st (A), major 3rd (C#), 5th (E) from the A mjor scale played together in some configuration.

If a chord uses a scale other than 1, 3 or 5, then it's identified by using that number in it's name.

So, if a chord is an A sus 2, then we know it consists of the 1st, 2nd instead of the 3rd, and 5th.

1st (A), 2nd (B), 5th (E)

Since you can play the A major scale in many different place & in multiple octaves we use numbers up to 13 in order to lable scale degrees for certain chords.

The A an octave higher is the 8th scale degree, so we keep going on from there.

1st (A), 2nd (B), 3rd (C#), 4th (D), 5th (E), 6th (F#), 7th (G#)

next octave higher

8th (A), 9th (B), 10th (C#), 11th (D), 12th (E), 13th (F#), 14th (G#)

You will see these intervals used in some discussion of theory. For example a major 3d an octave higher is a 10th. You will also see the 8th being used when indicating an octave higher in music notation: 8va.

However, tradionally we only use the 9th, 11th & 13th when labelling chords.

So, an Asus2 is 1st (A), 2nd (B), 5th (E).

An Aadd9 chord is 1st (A), 3rd (C#), 5th (E), 9th (B)

It's very difficult to play the B as a 2nd on the guitar, so it's usually played an octave higher.

Hope that helps!