Adding a 9th to a chord is the musical equivalent of performing an upgrade, but moreover, you’ll be in great company. The 9th is been a firm favourite with everyone from The Beatles to Oasis, and Kraftwerk to Bonobo.
Much like adding a 7th to a triad, adding a 9th is another form of extension, but one that builds upon the foundation of the chord which is already being used. It can also completely change the character of your harmony, particularly when coupled with the use of different types of 7th.
Before we consider these character definitions, let's work out how to find the 9th of your chord.
As with all extensions, the number indicates the degree of the scale. As a regular diatonic major or minor scale only includes 8 notes (say from C to C), the 9th is the next note in sequence, so in the case of C major scale, the 9th would be the note D, one octave and a tone above your key/starting note.
If we write a chord as a 9th chord, by default the implication is that a 7th will be included as part of its construction.
It's important to note that this flavour of 7th is the flattened variety, sometimes described as a dominant-style 7th. This means that in a chord of C9, a Bb will be included, along with the D above it.
This is quite a jazzy and bluesy sounding chord, and one that is popular in those genres, as well as rock.
It's also a chord shape that translates well to the guitar, which might explain its popularity with artists such as Nile Rogers.
If you think of the guitar break (in between verses) in the James Brown classic Papas Got a Brand New Bag, that's the power of a 9th chord, albeit in an inverted form.
For a far breezier flavour of 9th, try Cmaj9; once again, a 7th is applied by default, but the ‘Maj’ part of the chord symbol indicates something different this time.
We need the 7th degree of the major scale, which would be a B natural, with the addition of a D above it. It’s another favourite with players who are influenced by jazz and funk, but it's just at home in hits by artists such as Jessie Ware.
You can apply the 9th to other styles of chord, which are effectively mixed and matched from the two varieties of 7th chord.
At the more obvious end, you can simply construct a minor triad with a 7th above it, with the 9th on top.
You can also mix a match the minor triad, with major 7th. This will elicit a very mysterious chord, laced with spy-like character, and one that is enhanced further through the addition of the 9th.
However, if you get to the end of your verse and have landed on a dominant chord, you can create an amazing point of turnaround, by using the chord C7(b9). The makeup of this chord drops the 9th by a semitone (to Db, in our example), creating a brilliantly suspenseful turnaround point.
It works particularly well on a dominant chord, but can be used in other places, provided you feel it fits your harmonic structure.
The eagle-eyed among you may have noticed that the 9th is sometimes described as a 2nd. While the note itself might be the same note-name, the addition of the 9th extension, is often an indication of its placement in the chord, IE at the top of the chord, and not next to your keynote, as it might be in chord of Csus2.
The differentiation is subtle, and both chords offer similar harmonic tendencies.
As with all unique chordal elements or effects, use your 9ths sparingly, and you will find that they are all the more effective when you do deploy them.
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