Name the seventh

Four lit keys: a triad plus a seventh. Read it in two steps — the bottom triad first, then the distance from root to top.

A major triad with a minor 7th is a dominant 7th; with a major 7th it is a major 7th chord. A minor triad with a minor 7th is a minor 7th chord.

The diminished pair: a diminished triad with a minor 7th is half-diminished; with a diminished 7th (nine semitones) it is a full diminished 7th.

read the triad, then the top: maj+m7 = dom7 · maj+M7 = maj7 · min+m7 = min7 · dim+m7 = m7♭5 · dim+°7 = dim7

C major triad with a minor 7th on top: C7, the dominant.

Practise this in the trainer — it opens with this lesson the first time the concept comes up.