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, m7-5, 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.