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

Location:

• 3 semitones up from the 5th
• 2 semitones down from the root
• a tritone from the 3rd

Given that the guide tones (ie. the 3rd and dom7th) are a tritone apart, that means they are equadistant from each other. This is significant, because it means that any pair of guide tones serves as the 3rd and dom7th of two different roots

• ex. for both C7 and Gb7, the guide tones are E and Bb
• this is Function, basically meaning that it allows for the substitution of any dominant 7th chord with its tritone counterpart.

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)

We can easily form a minor 7th chord by making a triad 3 semitones up from our root

• ex. to make Cmin7, you simply play Ebmaj over a C root
  • therefore, Cmin7 is equivalent to Eb/C
  • conveniently, if we make it an Ebmin, we now have a C half-diminished chord

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

The 1-3♭-5-7 of the harmonic minor scale forms the minor major seventh chord

• ex. In C, has pitches C-E♭-G-B
• so-called due to the minor 3rd, and major 7th
• Works well in harmonic minor when played as the i chord.

infrequent in rock and popular music, is "virtually always found on the fourth scale degree in the major mode", thus making the seventh of the chord the third of the scale and perhaps explaining the rarity of the chord

Often when this chord is played, it is used as a passing chord.

• ex. in Something by the Beatles immediately following the line “I don’t want to leave her now”
  • NOTE: verify this. I'm skeptical..

Half-Diminished 7th (e.g. Cø7 or m7b5)

1-b3-b5-b7

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

Notably, the characteristic tritone between the 3rd and the dominant 7th is lost with a half-diminished chord

Another way to think of a half-diminished chord is as a minor 6 chord with the 6 in the bass

• ex. take Cø7. If we remove the C note, then we are left with a minor triad (built off of the minor 3rd scale degree of the root key, in this case Eb).
  • therefore, Ebmin/C is equivalent to Cø7

Examples

Diminished 7th (e.g. C°7 or G-7♭5)

1-b3-b5-bb7

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

• notably, the tritone between the 3rd and 7th (which are the characteristic guide tones of a dominant 7th chord) is retained in a diminished 7th chord. By diminishing the 5th, we then have to further flatten that flat 7th to retain the tritone (and thus, the tension)

This is a very jarring sounding chord

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

An interesting thing about diminished 7th chords is that they are symmetrical.

• ex. take a C°7 chord, which has notes C-Eb-Gb-A. If you build a diminished 7th chord from the Eb, you will end up with Eb-Gb-A-C. The pattern continues as we invert the chord again.
• since each chord can go by four different roots, that leaves only three distinct diminished 7th patterns.
• augmented triads are also symmetrical chords

Altered Dominant 7th

An altered dominant chord is a dominant 7th chord with an altered (ie. sharpened or flattened) 5 or 9 (or both)

Often used in jazz when the chord has a dominant function (ie. V-I perfect cadence)

• see