Construction
A 7#9 chord is an altered dominant 7th chord (1 - 3 - 5 - b7) where the ninth is raised by a semitone (#9) giving the formula 1 - 3 - 5 - b7 - #9.
You just have to add a ninth to the dominant 7 chord and raise it with a semitone to get a dominant 7#9 chord.
As it is shown in the chart below, by adding a 9 to a dominant 7 chord you obtain a dom9 chord and by raising the ninth you get a 7#9 chord.
| Dominant seventh chord |
1 |
3 |
5 |
b7 |
|
| Dominant 9th chord |
1 |
3 |
5 |
b7 |
9 |
| Dominant 7#9 chord |
1 |
3 |
5 |
b7 |
#9 (b3) |
7#9 chords have a blues sound because of the major third and the minor third. Indeed, as you can see in the chart above, the #9 can be considered as the minor third (b3).
This type of chord is often referred as the Hendrix (Jimmy) chord.
There is much of it in many jazz-blues compositions and very used by hard-bop musicians as Wes Montgomery (Four on six, Full House, D natural blues), Kenny Burrell (Chitlins con carne), Grant Green, Lou Donaldson and many more.
Notation Symbols
Dominant 7#9 chords can be written 7+9 or 7#9.
Related Scales
Minor pentatonic scale, minor blues scale, diminished scales or altered scale are generally used to play over 7#9 chords.
Basic 7#9 Guitar Shapes
Here are three main positions to play dominant seventh sharp ninth chords on the guitar.
As already mentioned before, 9th chords are theoretically made up of five tones, but it is physically difficult to play all of them.
That's why the fifth (5) is generally omitted. It is the least important notes which make up the 7#9 chord.