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6. 7th chords

By stacking thirds on top of each other, or taking one note every 2, you can create chords. But you don't have to stop at a triad, with 3 notes, you can add more.

When you add more notes to your triads, you're creating an extended chord.
If you add 1 extra note, you add the 7th, so that's a 7th chord.
If you add another extra note, you add a 9th, so that's a 9th chord.
Another extra note and you're adding a 11th, so that's called a 11th chord.
Another extra note and you're adding a 13th, so that's called a 13th chord. You get it.

If I try to make a 13th chord from C in the C major scale, that makes C E G B D F A. So technically this... is a 13th chord. Yeah!

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Again, the nature of these extended chords will depend on the intervals between each of their notes.
So let define all these intervals once and for all. 

INTERVALS

Every interval is define by the distance between a note of the chord and the root note.
Some intervals will be called perfect. If they're extended by a semi tone they're called augmented, and if they're shortened by a semi tone they're called diminished.

Then some intervals are either major or minor, with a semitone between the two states. If a major interval is extended by 1 semi tone it becomes augmented, and if a minor interval is shortened by 1 semi tone it becomes diminished. But these are more rare instances.

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To know which intervals are perfect or major/minor, just remember the C major scale, as everything seems to be built around that. That's all the white keys on the keyboard.

In the C major scale the fourth and the fifth are "perfect intervals" and the second, third, sixth and seventh are all major intervals.
From there it gives you all the reference intervals you need, to know your sixth or seventh is minor or major.
From there, it gives you all the reference intervals you need to know if your 6th or 7th is minor or major.

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For example, if you have 4 tones between two notes, that's like the interval between a C and a G# or Ab, so that's an interval of either an augmented 5th or a minor 6th.

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For the 7th, it is a M7th if the note is 5,5 tones above the root note. That a semi - tone below its octave. Or if it is 2 tones above the perfect 5th. It becomes a m7th F it's a whole tone below the root note, or 1,5 above the perfect fifth.

We can also observe that the major chords in this scales are built upon the perfect intervals, and the minor (or diminished) chords are built upon the non-perfect intervals.

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This figure can come in very handy as a reference, and should definitely be on a cheat sheet if you need one.

So now that we know how to build a 7th chord - by adding a 7th on top of a triad - and we know how we can identify different intervals, let's see the most used 7th chords.

7th CHORDS:

A major triad with a minor 7th (which would be a minor 3rd above the fifth) is probably the most common 7th chord and is called a "dominant 7th chord", because it is often used on the Vth degree, which is the dominant degree.
For a C chord that will make C E G Bb.
It would be noted C7

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If we have a major triad with a major 7th, that would make a major 7th chord.
So for C that would be C E G B and would be noted CM7

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