I spent many years teaching guitar, and all these tips are fairly practical and can be applied to many styles of music.
There are several chord types that include a 9th interval. Generally, you can replace simpler chords with some type of 9th chord, and it will improve the sound, with a more complex and interesting harmony. The important thing is - 9th and add 9 chords are different, and have different musical functions - don't mix them up.
Let's start with a C major scale: C D E F G A B C (eight notes)- the ninth note would be a D, as the sequence starts again. Therefore, C add9 is the notes C E G D. It's a nice chord, and will often replace C in the key of G. In other words, if the chords are G,C,D you can use C add9 instead of the plain C chord.
C9 is musical shorthand. What isn't very obvious is that it will include a flat 7, Bb in this case.
In practice, most C7 chords can be replaced with C9, giving you a more jazz, blues or funk sound. If you're looking at a chord chart for any song, where it has C7 you could use C9 or C13, as they are largely interchangeable. Sometimes a plain C7 will be better, especially for older styles of music such as New Orleans jazz.
Cm9 - almost the same shape, but the 3rd is minorised.That is, the 3rd interval in the chord is flatted by one fret or one semitone. Any major chord can be changed to a minor chord by changing this one note -a really useful concept if you don't already know it.
This is a great and widely used chord shape that can be moved up the neck - the root note is the one on string 5.
Can be used almost anywhere instead of a m7 chord. So a 2-5-1 chord progression in C could be Dm9 G7 Cmaj7.
9 Chords on Guitar
Efficient practice tips
Looking at the first three chord examples: the root note of each chord is on string 5. So, if you memorise the sequence of notes going up this string, you learn 11 different chords for the 9th shape and also 11 different chords for the min9 shape. Maybe start with the most commonly used chords, fret 5 will be D, fret 7 will be E, fret 8 will be F, fret 10 will be G
Minor 9 chords
Em add 9 and Am add9 are easy to play, but sound great, especially if you arpeggiate the notes, playing them one at a time from 6 to 1st string or 5th to 1st string. Could be used to enhance the sound of Em to Am.
A add9, D add9 and E add9 are the sound of The Police and Steely Dan - think Rikki Don't Lose that Number, a great song.
If you find the theory stuff too technical, just try the chords out and find a place for them in the context of a song you like, or try writing a song using them. There's a place for just enjoying the sounds without thinking about the theory.
Jon Green (author) from Frome, Somerset, UK on September 15, 2010:
Thanks Blair. The main thing is to use them in place of 7ths.
blair on September 14, 2010:
This is really helpful. Another helpful article on 9 chords is at http://guitarthinker.com/2010/09/difference-betwee...