Dansm's Guitar Chord Theory
Seventh Chords
Chords are the basis of all Western music. They are absolutely crucial to the guitar and to rock music, and an understanding of chords will improve your understanding of the guitar and music immensely. This page is designed to help you with the music theory behind seventh chords, which confuse a lot of people. So I hope this is helpful to all you beginners out there. Before you read this, you should be familiar with the theory of major and minor chords.
Seventh chords consist of a note added to the top of a major or minor chord. You can also add a note to the top of a diminished chord, but these rarely appear in popular music so I will ignore them. For example, your basic E chord consists of E, G#, and B. If you add a minor seventh to an E (the minor seventh of E is D), you have an E7 chord. The seventh chord follows this pattern: 1-3-5-m7. E7 chords appear most often in the key of A, because the E7 leads strongly back to the tonic A. Remember that in the key of A, E7 is called the dominant seventh (V7). An E7 chord is made up of these notes:

Why is it called a seventh chord? In the A scale, D is the seventh note above E. Count this on the diagram above. To use another example, a D7 would appear in the key of G. The note seven notes above a D is C, so a D7 contains D, F#, A, and C. Notice that the interval between the D and C or G and F is also a minor seventh (four whole steps and two half steps).
Common seventh chords are fingered as follows. To make any barre chord seventh,
check out my barre chord theory page.

dansm x02020 or x02223
x24242 or x21202
020100 or 022130

You can also add sevenths to minor chords. For example, a Bm contains B, D, and F#. To add a seventh, you use the minor seventh of B: A. So a B minor seventh (Bm7) contains B, D, F#, and A, as shown below. The minor seventh chord follows the pattern 1-m3-5-m7.

dansm x02010
022030 or 020000

The last type of seventh you will encounter is the major seventh chord. Major seventh chords are similar to normal seventh chords except they contain a major seventh (five whole steps and one half step) instead of a minor seventh. An Amaj7 chord would contain A, C#, E, and G#, as shown below. As you can see, major seventh chords follow the pattern 1-3-5-7.

It may be easier to think about a major seventh being one half step below an octave: the major seventh of A is G#, and the major seventh of G is F#. You don't come across major seventh chords too often but it is useful to know these:

dansm x02120
3x0032 or 3x0002

That concludes seventh chords. I hope this has been helpful in understanding how seventh chords are formed and why they look like they do. If you want, check out my page on 5 (like D5), 6 (G6), and 9 (B9) chords.
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1997 Daniel E. Smith.