The chord of the 9th—not at all a rare chord in 19th, 20th and 21st century music—is interesting. I was just thinking of the song 'Close 2 You', made popular by The Carpenters, and I realized that I had not often talked about harmony from the viewpoint of amateur musicians, especially guitar players. The very first chord of that song is a subdominant 9th. To avoid horrible terms like 'subdominant,' we have to decide what key the song will be in; let's say F. The most common chords of F major are
(1) F-A-C, the F chord;
(5) C-E-G, the C chord;
(4) B♭-D-F, the B-Flat chord.
But the first chord of this song is, stripped down, B♭-D-F-C.
Hey, you're probably thinking, what's that C doing there?
Well, before the chord sounds, there are two notes: D F, which correspond to the words
"Why do ..."
The next word is "birds," and that word is sung to C, which is why the chord of the 9th makes sense here.
The song continues with "...suddenly appear, anytime you are near?"
There is an excellent reason why this chord works well there; it's because the notes to which the next few syllables are sung are B♭ C and D, so we would consider the C an (accented) passing note, which are notes that are not harmonized; they're just sounded over the preceding harmony. The rules of popular music have evolved to allow those passing notes to be incorporated right into the chord, and it's given a name: chord of the (added) ninth. (On B♭, of course.)
