As we know, triads are made up of a root, a third above that root, and a fifth above that root. Seventh chords add ​a seventh above the root. If the notes are written as close together as possible in root position, they will still look like a snowman.

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What's a Seventh Chord?

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Naming Seventh Chord

Like triads, seventh chords are named for their triad quality (e.g. major, minor, augmented, and diminished) and their root (e.g. A, Bb, F#). However, they are also named for the quality of the seventh above the root.

Look at the picture above. Each member of the G major scale has a third, fifth, and seventh added. Notice that there are no accidentals added outside of the key signature. This means that these are the diatonic seventh chords in a major key.

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If we look at the quality of the triad written above scale degree 1, we see the notes G B D F#. Together, G B D forms a major triad. When we look at the relationship between the seventh and the root (​G to F#), we can see that is a Major 7th. That means the quality of this seventh chord is a G major-major 7th. We typically shorten that name to a G major 7th chord.

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Major

Seventh Chords in Major Keys

Let's look at a different quality now. If we look at the quality of the triad written above scale degree 2, we see the notes A C E G. Together, A C E forms a minor triad. When we look at the relationship between the seventh and the root (​A to G), we can see that is a minor 7th. That means the quality of this seventh chord is an A minor-minor 7th. We typically shorten that name to an A minor 7th chord.

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Minor Seventh Chords in Major Keys

But wait, there's more! If we look at the quality of the triad written above scale degree 5, we see the notes D F# A C. Together, D F# A forms a major triad. When we look at the relationship between the seventh and the root (​D to C), we can see that is a minor 7th. That means the quality of this seventh chord is a D major-minor 7th. We typically shorten that name to a D dominant 7th chord.

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Dominant Seventh Chords in Major Keys

But... why "dominant" seventh?

There is only one dominant seventh chord in a major key. It is always built on scale degree 5. Remember, the special name for scale degree 5 is dominant. ​This is why the major-minor 7th chord is referred to as a dominant 7th chord.

Okay, last one for now. If we look at the quality of the triad written above scale degree 7, we see the notes F# A C E. Together, F# A C forms a diminished triad. When we look at the relationship between the seventh and the root (​F# to E), we can see that is a minor 7th. That means the quality of this seventh chord is an F# diminished-minor 7th. We typically change that name to an F# half-diminished 7th chord.

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Half-diminished Seventh Chords in Major Keys

If all of the notes are diatonic (in the key), the seventh chords of a major key will match the picture below. We still indicate triad qualities by making our Roman numerals upper case or lower case, but now we add a 7 after to indicate the presence of an added seventh.

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Seventh chords built on scale degrees 1 and 4 are major sevenths (MM7). Seventh chords built on scale degrees 2, 3, and 6 are minor sevenths (mm7). Scale degree 5 is the only one that forms a dominant seventh (Mm7), while scale degree 7 alone forms the half diminished seventh (dm7). Notice the slash through the ^o for half diminished.

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Diatonic Seventh Chords in Major Keys