The name gives it away: seventh chords chords must have some kind of
seventh in them. Indeed, they are triads bringing their plus-one. Here
are some examples with root G:
The first of these examples, the G△ is a G major 7. In this
chord symbol, G signifies that the base triad is G major, and the
△ specifies the character of the seventh -- this one is a
major seventh, as opposed to a minor seventh. Read
1-intervals{.interpreted-text role="doc"} first if you don't know
what this means. Here are three examples of major seventh chords:
G△D△B♭△=⎩⎨⎧G major triadg,b,d+major seventhf♯⎭⎬⎫=⎩⎨⎧D major triadd,f♯,a+major seventhc♯⎭⎬⎫=⎩⎨⎧B♭ major triadb♭,d,f+major seventha⎭⎬⎫
The same principle applies to the other seventh chords. A
m7-chord is made up out of a minor triad and a minor seventh.
Gm7Dm7B♭m7=⎩⎨⎧G minor triadg,b♭,d+minor seventhf⎭⎬⎫=⎩⎨⎧D minor triadd,f,a+minor seventhc⎭⎬⎫=⎩⎨⎧B♭ minor triadb♭,d♭,f+minor seventha♭⎭⎬⎫
Here are some examples of 7-chords, known as dominant chords for
reasons that will become clear in the next post about chord
progressions.
G7D7B♭7=⎩⎨⎧G minor triadg,b,d+minor seventhf⎭⎬⎫=⎩⎨⎧D minor triadd,f♯,a+minor seventhc⎭⎬⎫=⎩⎨⎧B♭ minor triadb♭,d,f+minor seventha♭⎭⎬⎫
You should be able to assemble the other seventh chords yourself, based
on the information in this table:
Name
Notation example
Base triad type
Seventh type
Major seventh
C△
Major
Major
Dominant
C7
Major
Minor
Minor seventh
Cm7
Minor
Minor
Minor major
Cm△
Minor
Major
Diminished
Co
Diminished
Diminished (enharmonically equivalent to M6)
Half-diminished
Cø
Diminished
Minor
And this decision tree may help you structure the various types of
seventh chords into categories in your mind. Don't worry if it the
information does not seem very actionable right now, the next part about
chord progressions will help a lot with that.