This course/website/book focuses on a particular danceable blues style: jump blues and swing blues.
In thse styles we commonly use ‘dominant seventh chords’.
A dominant seventh chord has a chord formula : 1 – 3- 5 – b7
To build this chord we need
a root (note 1),
a major third interval up from the root (3) = 4 frets up,
a perfect fifth up from the root (5) = 7 frets up
and a flatted seventh (b7) = 10 frets up.
Each fret is of course 1/2 note.
If we were to do this on a G root, we would end up
with the notes G, B, D an F.
These notes together form a G7 chord.
The shapes of these chords can vary.
We have a LOT of different domiant 7th shapes available to us on the guitar.
All these shapes have in common that they have a root, a major third, a perfect fifth and a flatted seventh: 1 – 3 – 5 – b7.
Look at the overview of chords at the back of this book/website for some examples.