Adding a seventh

So far we've talked mostly about two types of chords, major and minor triads. These chords are build of a root note, a major or minor third, and a perfect fifth. (If you need a reminder of what these musical intervals are, start here.)

These three-note chords predominate in rock songs, but four-note seventh chords are also widely used. To turn a triad into a seventh chord, just add a note a third above the fifth (or a second below the root.)

Remember our six diatonic triads? Let's add a seventh to each of them and take a look at our six diatonic seventh chords. In C major:

There are three minor seventh chords, with the m⁷ suffix. These are composed of p1 + m3 + p5 + m7.
[six]m⁷, [two]m⁷, [three]m⁷ — Am⁷, Dm⁷, Em⁷

There are two major seventh chords, with the maj⁷ suffix. These are composed of p1 + M3 + p5 + M7.
[one]maj⁷, [four]maj⁷ — Cmaj⁷, Fmaj⁷

There is one more diatonic seventh chord, with the ⁷ suffix. This is simply called a seventh chord, and to avoid confusion you can call it a common seventh or dominant seventh chord. And this chord is, of course, [five]⁷ — G⁷

Three m⁷ chords, two maj⁷ chords, but only one ⁷ chord. This chord's uniqueness gives it a special role. It's the only chord we've covered so far that can, all by itself, tell you exactly what key you're in. Because, unless non-diatonic notes are complicating the picture, the keynote of a major key is always a perfect fifth below the root of any ⁷ chord you hear.

This unique feature of the ⁷ chord is very handy when you need to change from one key to another. No matter what chords you've wandered through, if you land on a ⁷ chord, people will probably hear it as [five]⁷ and mentally adjust to your new key.

There are other possible types of seventh chords if we acknowledge the usually-overlooked [seven]dim triad and allow for chromatic (non-diatonic) notes.

The diminished seventh chord, dim⁷ or °⁷, is composed of p1 + m3 + d5 + d7. A d7 (diminished seventh) interval is a half-step smaller than a m7, and is equivalent to what we usually call a M6.

The minor seventh with lowered fifth chord, m^7-5, is composed of p1 + m3 + d5 + m7. This is a diminished triad with m7 added.

The minor with major seventh chord, m(maj⁷), is composed of p1 + m3 + p5 + M7.

The seventh with lower fifth, ^7-5, is composed of p1 + M3 + d5 + m7.

The seventh with raised fifth, ⁷⁺⁵, is composed of p1 + M3 + A5 + m7.

Tuesday is Beastly Fundamentals Day.