Adding an additional third on top of a root position triad forms a seventh chord, as this new note will be a seventh above the root.

Although a seventh can be added to any chord, by far the most common seventh chord is V⁷, with ii⁷ in a distant second place.

In a minor key signature, the ii chord becomes a half-diminished seventh. (The chord is considered half-diminished because the interval from the fifth to the seventh is a major third. In a fully diminished seventh chord, all three intervals would be minor thirds.)

Note that the leading tone is raised in the example below to create a harmonic minor scale. This affects the quality of the III-chord, V-chord, and vii-chord.

A seventh is a tendency tone, meaning it leads to another pitch. Sevenths tend to push down, and so the seventh of a V-chord (scale degree 4) pushes down to scale degree 3. Since the V-chord also has a leading tone, which pushes up to the tonic, V⁷ leads very strongly back to the I-chord.