Every chord we use in blues or any other type of music is derived from a scale.
The chords we used in our first example G blues (G7, C7 and D7) are “dominant seventh” chords.

These are derived from a Mixolydian scale.

This scale has seven notes and has this formula :
2, 2, 1, 2, 2, 1, 2.

G Mixolydian

As you can see, it looks a lot like a regular major scale.
The only note that’s different is the F, a flat seventh up from the tonic.

A Mixolydian scale sounds a certain way because of the intervals (number of frets) between the notes.

This is the case with every Mixolydian scale.
If you find a scale that has these intervals, it’s a Mixolydian scale and it’ll sound this way.

If you want to play a C or D Mixolydian scale, find the tonic on the first string and play the pattern in the diagram above.

That didn’t hurt, now did it?