Seventh

Dominant 7th (e.g. C7)

• aka Major-Minor 7th Major triad with a minor 7th (ie. of the minor scale; 2 semitones below octave)

Here, dominant refers to how it's used (function), unlike the other 7th chords which refer to how the chord is made (ie. by using a 7th or flattened seventh)

Usually built on the fifth degree of the major scale

Dominant seventh chords contain a strong dissonance— a tritone between the chord's third and seventh.

The dominant seventh is perhaps the most important of the seventh chords. It is useful because it contains both a major triad and the interval of a tritone.

The V7 chord is found almost as often as the V, the dominant triad, and typically functions to drive the piece strongly toward a resolution to the tonic of the key.

Because of this original usage, it also quickly became an easy way to trick the listener's ear with a deceptive cadence. The dominant seventh may work as part of a circle progression, preceded by the supertonic chord, ii.

Importantly, non-diatonic dominant seventh chords (sometimes called a chromatic seventh), borrowed from another key, can allow the composer to modulate to that other key.

In popular rock and roll songs following the blues progression, the IV and V are almost always dominant 7ths

Major 7th (e.g. Amaj7)

Major triad with a major 7th (ie. of the major scale; 1 semitone below octave)

Minor 7th (e.g. F♯min7)

Minor triad with a minor 7th (ie. of the minor scale; 2 semitones below octave)

Minor-Major 7th (e.g. CmM7)

Minor triad with a major 7th

In C, has pitches C-E♭-G-B

Works well in harmonic minor when played as the i chord.

Half-Diminished 7th (e.g. C°7)

Diminished triad with a minor 7th (2 semitones below octave)

This is known as half diminished because if we split this chord up into 2 halfs (the triad-side and the seventh-side), only one of the halves is diminished (the triad half), while the seventh half is still just a minor 7th.

Though the chord is more commonly used to increase dissonance in a more minor way, it can also be used to express more uplifting emotion, as is found in the first chord (following the fanfare) of Mendelssohn's Wedding March

Examples

Diminished 7th (e.g. Cø7 or G-7♭5)

Diminished triad with a diminished 7th (3 semitones below octave)

This is a very jarring sounding chord

• "diminished sevenths were to feature regularly in music to evoke the uncanny or sense of impending danger."