You may hear musicians refer to the dominant seventh chord. This does not refer to a chord that should come seventh on the list of the most important chords in any given key.
If refers to the fifth chord of the key once it’s been altered a little.
This is done by taking the seventh note of the key, flattening it a semitone and then adding that note to the chord. If that sounds like gobbledegook don’t worry about it. All you really need to know is that the dominant seventh chord is based on the fifth chord of the key and it is very important because it used a great deal.
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If you look at the first line of the chart you will see that the fifth chord in the key of C Major is G and next to it we have listed G7. This G7 is the dominant seventh of the key of C. Look down the chart and you will see the dominant seventh in the other keys.
We’ve now got used to the sound of dominant seventh chords because they are used so much – not only in blues and jazz but also in folk music.
That’s a relatively recent development in taste, however. Before, the 20th Century, many people frowned on the use of the dominant seventh as it sounded discordant to them.
Nowadays, it sounds perfectly familiar and we hardly notice it.
The dominant seventh is so named because it is the most important chord in the key except for the root chord.
The fourth chord is the next most important which is why it is sometimes referred to as the sub-dominant chord.
So to recap, the first chord is the root or key chord, the fifth chord is the dominant chord and the fourth chord is the sub-dominant.
The reason the first, fourth and fifth chords are so important and can be used to harmonise any song is because between them, they contain every note of the major scale.
The root chord has the first, third and fifth notes of the scale.
The fourth chord has the fourth, sixth and first notes of the scale.
The fifth chord has the second, fifth and seventh notes of the scale.
The fact that you can harmonise virtually every song with these three chords doesn’t mean that you should.
Many songs only contain these three chords, but many others will sound much better if you add other chords which will add to the texture and the effect.
Learn as many chords as you possibly can. Some chords you may only use occasionally, but when that occasion comes, they will help to make your playing much better and much more satisfying.
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