Adding an additional note to a triad (chord) creates a tetrad. This additional note can alter the flavor of a triad in surprising and interesting ways. The most common tetrad is the seventh chord.
There are eight different kinds of seventh chords, each displayed below. I have provided formulaic explanations and three examples (fretboard diagrams) for each.
(1) A major seventh added to a major triad is a major seventh chord. Major seventh chords have a dreamy and stable character:
(2) A minor seventh added to a major triad is simply called a seventh chord. This construction is sometimes also referred to as a dominant seventh chord. Dominant 7th chords are the most commonly used tetrads, as they provide a strong tension that pleads for resolution, often to the tonic triad:
(3) A major seventh added to a minor triad is a minor/major seventh chord. Often a transitional chord, it has an air of mystery:
(4) A minor seventh added to a minor triad is a minor seventh chord. Minor sevenths are to the minor triads what the major seventh is to major ones—a stable substitution that adds flavor to the diatonic triad:
(5) A minor seventh added to a diminished triad is a minor seventh flat five chord. This chord is also known as a half-diminished seventh chord. Half-diminished seventh chords are often used as substitutions for minor triads in jazz:
(6) A diminished seventh added to a diminished triad is a diminished seventh chord. In much of Western music, it serves the same function as the dominant seventh chord:
(7) A minor seventh added to an augmented triad is an augmented seventh chord. This chord is considered an altered dominant, similar in function to a dominant seventh chord, but with added tension:
(8) A minor seventh added to a major triad with a flat five is a dominant seven flat five chord. The dominant seventh flat five is another altered dominant:
There are, of course, other four note chords you can create using the notes in the major scale—beyond which there are chords with five, six, and even seven notes! Seventh chords, though advanced enough for this book’s purposes, are only the tip of the iceberg. You may want to play around with chord structures on your guitar to see what you can come up with.
Here are the seventh chords of a major scale in the key of C major. The root notes of the following chords are all played on the A-string: