The simplest way to think of a seventh chord is three thirds stacked on top of each other.

The dominant seventh is the most important of the seventh chords and is fundamental to Western harmony. A useful shortcut is to think of a dominant seventh chord as a major triad with a minor third on top.

If we play a G major triad, for example:

… we can add the note a minor third above the top note:

This forms a G dominant seventh chord. It’s often called G7 for short.

More precisely, a dominant seventh is a chord that consists of a major third, perfect fifth, and minor seventh above a root.

Let’s try another example. If we take C as our root, a major third above C is E:

A perfect fifth above C is G:

If we combine a major third and perfect fifth above the same root, we get a major triad:

Finally, a minor seventh above C is B-flat:

If we add our minor seventh to the other notes, we get a C dominant seventh chord:

Major-minor seventh

The dominant seventh chord is sometimes also called a major-minor seventh. “Major” refers to the major third, while “minor” refers to the minor seventh. (The perfect fifth is assumed.)

Terminology

What’s the difference between a “C dominant seventh” and the “dominant seventh of C”? The terminology can be rather confusing, so let me clarify some frequent terms you’re likely to encounter.

C dominant seventh, C major-minor seventh and C7 all refer to the same chord:

This chord is built on the note C, which means that C is the root of the chord. (Even if C is not the lowest note—in other words, if the chord is inverted, as long as we have exactly this combination of notes, C, E, G and B-flat, C is still the root of the chord.)

Importantly, the C7 chord is independent of key: You could play in any key, with any key signature, and if you encounter the notes C, E, G and B-flat played as a chord, you’ll play a C7 chord.

Dominant seventh of C refers to a different chord, however. This refers to the dominant seventh chord in the key of C, or the dominant seventh chord that resolves to C. “C” can be C major or C minor: It doesn’t actually matter because the dominant seventh of C major is the same as the dominant seventh of C minor.

“Dominant” refers to the fifth scale degree, in other words, the fifth note of the scale. Let’s continue using the scale of C major. The notes of the C major scale are C, D, E, F, G, A, B. The fifth note of the scale is G.

If we build a chord on scale degree 5—in this case G—using only the notes within the scale, we play a dominant chord, also called dominant triad:

This chord is still a triad because there are only three notes.

If we then add the seventh from scale degree 5—in other words, the note in the C major scale that is a seventh away from G—we now get the dominant seventh of C:

To clarify, here are some terms you might encounter:

C dominant seventh is the dominant seventh chord built on C. It consists of the notes C, E, G and B-flat:

The dominant seventh of C refers to the dominant seventh chord that resolves to C (either C major or C minor). It is built on the dominant of C, in other words, the fifth note of the C major (or C minor) scale, G. It consists of the notes G, B, D and F:

Summary

As a summary, just remember two things:

1. There is a simple shortcut for building a dominant seventh chord on any note: Play a major triad, then add a minor third on top. These are the notes of the dominant seventh chord.
2. The dominant is the fifth note of any scale. If we build a major triad on this note, we’ll get the dominant chord, also called the dominant triad. If we use only the notes of the scale and add the seventh—meaning the note a seventh above the dominant—we’ll get a dominant seventh chord. An easy shortcut is to start on the fifth note of a scale, then play a total of four notes, skipping over every other note.

With these shortcuts you can now build a dominant seventh chord on any note and find the dominant seventh chord in any scale.