Here is a short instrumental I recorded in March 2017, I don’t think I “released” this on to SoundCloud or BandCamp at the time. I have now EQ’d it (basically put in a low cut) and added a little splash of reverb and some stereo widening to make it sweeter and more open to listen to.
It’s called Cadd9, because that’s basically the chord on which the whole composition hinges. Nothing much to it, to be honest.
The chord Cadd9, is the C major chord with an extra note from the scale of C major, the 9th note, perhaps obviously (or the second note, if you like), the D. The chord is thus the root note C, the major third note, E, the perfect fifth note, G, and then you add that D, the 2nd or 9th note of the C major scale depending on your perspective.
Cadd9 is not to be confused with C9. C9 is built by adding the 9th note to the dominant 7th chord rather than the major chord. The C (dominant) 7th has an A# (or Bb) in it, so C9 has the notes CEGDA# and has an altogether different colour to the Cadd9.
Of course, to confuse things a little more there are also chords that have a 2nd (or 9th) in them, Csus2 (C suspended second) but lack the major third note. So, Csus2 has only the following notes C, G, and D, and in one sense, it’s not a C chord at all, it is a Gsus4.