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A seventh chord is a type of chord that is built by taking a triad and adding an additional note: the 7th note of the scale. For example, if we took a C major triad and added the seventh note of C major scale (B) we would then be playing a C major 7 chord:
In music, there are five different types of 7th chords. Each of these types uses different intervals between the notes in the chord which gives it a different sound and feel:
Diminished 7th
Major 7th chords are built by taking the major triad and adding a 4th note which is a major 7th interval about the root note. A C major 7th chord would use the notes C, E, G and B:
There are a few ways to write a major 7th chord as a chord symbol. You might see it written as whatever note is the tonic of the chord + M7, Ma7, Maj7, or Δ.
A dominant 7th chord is built by taking the major triad and adding a 4th note which is a minor 7th interval about the root note. In C, this would be C, E, G and Bb:
A dominant 7th chord symbol or the way of writing it in shorthand is to just have the tonic note of the chord followed by a 7.
Minor 7th chords: Instead of using a major triad, these are built using a minor triad with a minor 7th interval. In C this would be C, Eb, G and Bb.
Minor 7th chords tend to be written using a lowercase m with a number 7 or by writing min7 after the tonic note.
Half diminished 7th chords are built by combining a diminished triad chord with a minor 7th interval which in C would be C, Eb, Gb and Bb.
Half diminished 7th chords are typically notated as a small circle with a line through it followed by a 7 like this: ø7. You could also see it written as 7 b5 which means the same thing.
Diminished 7th chords are built using a diminished triad with a diminished 7th interval with every interval between each of the notes being 3 semitones apart. In C this would be C, Eb, Gb and Bb.
To notate a fully diminished 7th chord you’ll often see it written as °7 or by writing dim7.