Guitar Chord theory

Major and Minor Triads

The most basic kind of chord is called a triad, because it boils down to just three notes. For example, an A-major chord consists of A, C#, and E–notes that are referred to as the root, major third, and fifth of the chord. You can also have a minor triad; an A-minor triad, for example, consists of A, C, and E, which are the root, minor third, and fifth of the chord. The only difference between a major triad and a minor triad is the third. Every major third in every major chord is two whole steps (four frets) above the root of the chord. Every minor third in every minor chord is a step and a half (three frets) above the root of the chord.

Augmented and Diminished Triads

What happens to a triad if the fifth changes? The fifth found in both the major and minor triad is called a perfect fifth. There are three-and-a-half steps (seven frets) between the root and a perfect fifth. If you take an A-major triad and raise the fifth a half step to E#, you have the interval (A to E#) of an augmented fifth, and the resulting chord–A, C#, E#–is called A augmented.

Going in the opposite direction, if you take an A-minor triad and lower the fifth by a half step, you’ve got the interval (A to Eb) of a diminished fifth, and the resulting chord–A, C, Eb–is called an A diminished.

So we’ve got four kinds of triads: major, minor, augmented, and diminished. Here they are in terms of their roots, thirds, and fifths

R	R	R	R
3	b3	3	b3
5	5	+5	b5
Major	Minor	Augmented	Diminished

Sevenths

In a scale or chord, a seventh is the seventh note up from the root. Looking at an A-major scale, the seventh is G#. From A to G# is a major seventh, or an interval of five-and-a-half steps (11 frets). A minor seventh is one half step smaller than a major seventh, or an interval of five whole steps (ten frets). For example, a minor seventh up from A is Gn. As a shortcut when dealing with chords, you might try thinking of the major seventh as just a half step (or one fret) below the root and the minor seventh as a whole step (or two frets) below the root.

The various kinds of seventh chords you may have seen are just triads with an interval of a seventh up from the root added on. The question is, what kind of seventh–major or minor–is being added to what kind of triad: major, minor, augmented, or diminished?

To keep things relatively simple, let’s leave the augmented and diminished triads out of it for now and stick to just major and minor triads and sevenths. You’ve got four possibilities: major triad with major seventh, major triad with minor seventh, minor triad with major seventh, and minor triad with minor seventh.

There’s a different name for the result of each one of these combinations. Two of them are pretty logical. If you add a major seventh to a major triad, the result is called a major-seventh chord. Similarly, if you add a minor seventh to a minor triad, the result is called a minor-seventh chord. These are usually abbreviated maj7 and m7 (or min7). So when you see Amaj7, you can think to yourself, "Aha! Everything major–an A major triad with a major seventh on top." Since the seventh note of an A-major scale is G#, what you need is an A-major chord with a G# buried in the middle or perched on top.

Likewise, when you see Am7, you can unpack it by thinking, "OK, everything’s minor . . . an A-minor triad with a G natural added in." (G natural is a half step lower than the major seventh, G#, and therefore is the minor seventh of A.) Either one of the chords below would fit the bill nicely, with a G natural either insinuated into the midst of the chord or waving around conspicuously on the high end.

So far, so good. But now, alas, is where things get a little peculiar. What do you call a major triad with a minor seventh on top? Not a major-seventh chord, because the seventh itself is minor. And not a minor-seventh chord, because the triad itself is major. This hybrid of major and minor is called a dominant-seventh chord. Confusion can ensue quickly, because a dominant-seventh chord is generally referred to as simply a seventh chord: an A dominant-seventh chord, for example, is simply written A7, not Adom7. So you need to know a couple of things: when you see A7, it means A dominant seventh, and that in turn means an A-major triad with a minor seventh added, or an A-major chord plus a Gn, which would look like one of the two chords below:

The last possibility, a minor triad with a major seventh, is pretty rare, so we won’t worry about it too much. It’s also notated the most literally: an A-minor triad with a G#, or major seventh, added is written Am(maj7). Imagine that.

Going back to our diagram, then, we can fill it in like this:

	Major 7	Minor 7
Major Triad	maj7 chord	7 chord
Minor Triad	m(maj7) chord	m7 chord

From here, it's not so hard to start constructing these chords on other roots. For example, to create a Dmaj7, lower the D on the second string one fret and play the top three strings with one second-fret barre. To play an Em7, start with an E-minor voicing and lower the root on the fourth string two frets to the minor seventh. Since that note is the open D, you can play the resulting chord with just one finger.