Dorian

The note that defines it

D E F G A B C, then D. Compare with D natural minor, which has B♭ where this has B. That raised sixth is the whole difference, and it lifts the scale without making it major.

Because it uses only natural notes, D dorian is the easiest mode to hear and to play: it is the C major scale beginning in a different place, and no accidental is involved.

Why it is used

A minor chord with a major sixth above it is the sound of modal jazz, of a great deal of folk music, and of film cues that need unease without sadness. Miles Davis built an album on it.

On guitar the mode sits in first position with the open D as the root, and the shape at the fifth and tenth frets moves it up the neck without changing a note.

Where it shows up

Modal vamps: a bar of Dm7 repeated, with the melody drawing on the B natural to keep the harmony from settling into C major or D minor.

English and Irish traditional tunes use it constantly, often without naming it — a tune in D with a C natural and a B natural is in dorian whether or not anyone says so.

The simplest way to hear the mode is to play only white notes over a held D bass: nothing changes about the notes, and everything changes about where they seem to belong. That experiment is also the clearest demonstration of what a mode is.