So, what is a 7th chord, and how does it function? The basic idea is that you’re adding a note to a chord that is 7 letters above the root note.
OK…but how does it function? Well, when I say “function”, I really mean “how it works within a scale or a key”.
Harmonising a Scale
Basically, a “key” is made by taking the notes of a scale, and making chords out of them (I go into more detail in the page linked above). So, you’re in a major “key” when your song mostly uses chords that are made from the notes of the major scale.
Making sense so far? If not, feel free to follow the links to pages that should have the explanations (or comment below).
If we harmonise the C major scale, we get these chords:
C Dm Em F G Am Bdim
Each one is built from a note of the scale, and each chord has a different “function” within the scale. For example, if you play the 5th chord, G major, it will naturally want to resolve to the C chord- and that’s it’s “function” within the key. The B diminished chord will have a similar effect, and the C chord will feel like “home” and is often used at the end of a song (if the song is in C major, of course).
These types of chords are the same for every major scale (regardless of what note we start on), so to be more generic, we can just use numbers for these chords:
I ii iii IV V vi vii
Roman numerals are usually preferred, and the numbers in capitals are major chords, and the lowercase are minor chords.
7th Chords
The chord we’re interested in today, though, is the “dominant 7th”. This only occurs at one place in a major scale, and has a very unique sound and function.
Lets continue the process of harmonising this scale up to 7th chords (re-read the “harmonising a scale” page if you’re confused). We end up with this (we’ll start from a C note again so you can see the difference):
Cmaj7 Dmin7 Emin7 Fmaj7 G7 Amin7 Bmin7(b5)
See, there is only one place in a scale where the dominant 7th chord lives. The fifth chord (or the “G7” in our example) is the dominant 7th chord, and (as I kinda mentioned earlier) it wants to resolve to the “I” chord (which is a “C major”, or “C major 7” in our example).
So, if you have a chord progression where you have a dominant 7th chord, which resolves to a major chord that is 7 semitones/frets (the distance between C and G) lower, that’s called a “functioning dominant” chord.
Here’s how those two chords look in tab:
G7 C
e --3-------3------
B --3-------5------
G --4-------5------
D --3-------5------
A --5-------3------
E --3--------------
Play it to see what I mean- it’s the classic “end of a piece” sound!
These two bar chords are a good example, because the can be moved anywhere on the neck. Here’s another “moveable” shape:
C7 F
e --3-------1------
B --5-------1------
G --3-------2------
D --5-------3------
A --3-------3------
E ----------1-----
This time it’s in the key of F, but you can move these shapes to whatever key you want.