Blues

The notes

D F G A♭ A C, then D. The A♭ is the blue note between the fourth and fifth degrees, and it works as a passing tone or a bend target rather than a resting place.

The scale contains F against a D7 chord that contains F♯. That collision between the minor third of the melody and the major third of the harmony is the definition of the idiom.

On the fretboard

Tenth fret with the root on the low E, or fifth fret with the root on the A string. Open position also works, since D, G and A are open strings, and that mixture of open and fretted notes suits country phrasing.

The open D string as tonic means it cannot be bent, so phrases in this position tend to resolve downward — the same constraint that shapes blues in E.

Where it is used

Blues and country in D, and rock riffs in drop-D tuning where the lowered sixth string doubles the root.

It is also the key many acoustic blues players prefer, because the open strings support the scale without the volume loss that a capo introduces.

It is also the scale that fits drop-D riffs without modification: lowering the sixth string to D puts the tonic under the thumb, and the box above it stays exactly where it was.