Chords with an Added 7th
A “7th chord” is a general term for any triad with an added note which is a 7th higher than the root of the triad.
Seventh chords have specific names, depending on what type they are.
The type of 7th chord will always depend on the type of triad plus the type of 7th from the root. The most common 7th chords are the following:
|7th from root
|Type of 7th chord
|Minor major 7th
|Half diminished 7th
We can build a 7th chord on each degree of the scale in any key (without using any accidentals). For example, in C major:
1st degree: C-E-G is a major triad, plus B is a major 7th above root= a major 7th chord
2nd degree: D-F-A is a minor triad, plus C is a minor 7th above root = a minor 7th chord
3rd degree: E-G-B-D = minor 7th
4th degree: F-A-C-E = major 7th
5th degree: G-B-D-F is a major triad plus a minor 7th. This chord is specifically called the dominant 7th.
6th degree: A-C-E-G = minor 7th
7th degree: B-D-F-A = is a diminished triad plus a minor 7th and is called a half-diminished 7th chord.
In minor keys, there are more combinations because the 6th and 7th degrees of the scale may or may not be sharpened, (see the melodic minor scale).
Here are the chords formed when just the 7th degree (leading note) of the scale is sharpened:
How 7th Chords are Notated in Pop Music
In pop music, chords (for guitar, piano, etc.) are notated like this:
7 means dominant 7th so C7 would be C-E-G-Bb
m7 means minor 7th so Cm7 would be C-Eb-G-Bb
M7 or Maj7 means major 7th so CM7 would be C-E-G-B
The Power of the Dominant 7th Chords
Dominant 7th chords have a VERY strong function in Classical harmony – that particular combination of notes leads very strongly towards the tonic chord. Whatever key you are in, if you introduce a chord of that type it will SOUND like a dominant 7th, even if it’s not actually built on the dominant (5th degree of the scale).
There is no other chord (other than the dominant) which uses that combination of intervals, so if you want to achieve a chord with the same intervals which isn’t built on the dominant, you’d have to introduce an accidental (or 2).
Once you have introduced an accidental, you are making the music change key. If you wrote E7 in the key of C major, you’d expect the key to then change to A minor (or major). The key change can last just a bar or so, or for a long time. But it’s still a change of key.
In jazz and other modern styles, it’s common to have a succession of dominant chords which never resolve to the tonic.