How to build chords from scales
Scales and chords are related and chords are “built” from scales. Knowledge about scales is great for your understanding of chords and vice versa. By understanding their relationship, you don’t always have to memorize all the tones in different chords, instead you can rely on your knowledge to figure out by which tones a chord is constructed.
The Major and Minor scales are the foundation for most chords
By knowing the Major Scale and the (Natural) Minor Scale you will be able to envision a large part of all chords. Chords are constructed by tones that are included in these scales and ordered by certain intervals.
Chord categories based on the Major Scale
These are some chords built on the C Major Scale (C, D, E, F, G, A, B):
All of these uses C note as the root and thereupon more tones from the C Major Scale:
C: the root (C), major third (E), perfect fifth (G).
Cmaj7: the root (C), major third (E), perfect fifth (G), major seventh (B).
Cmaj9: the root (C), major third (E), perfect fifth (G), major seventh (B), major ninth (D).
These are far from all chords that can be built from a C Major Scale, but some of the more common. In cases there the ninth is used as an interval, the chord includes tones from two octaves (an octave consists of eight tones).
If you wonder about names like major third, major fifth, perfect fifth and so on, this is terminology of different intervals.
The lesson from this is partly that you can use the same intervals and creating chords in other keys based on that particular major scale. For example, from the D Major Scale the corresponding D, Dmaj7 and Dmaj9 chords can be built with the same intervals.
All major seventh chords, for example, are built with a root, a major third, a perfect fifth and a major seventh. What differs is the tones depending on the scale.
Chord categories based on the Minor Scale
These are some chords built on A Minor Scale (A, B, C, D, E, F, G):
All of these uses A note as the root and thereupon more tones of the A Minor Scale:
Am: root (A), minor third (C), perfect fifth (E).
Am7: root (A), minor third (C), perfect fifth (E), minor seventh (G).
Am9: root (A), minor third (C), perfect fifth (E), minor seventh (G), major ninth (B).
Similar to the example C Major Scale, the same intervals are used for creating chords in other minor keys based on particular scales. For example, from the B Minor Scale the corresponding Bm, Bm7 and Bm9 chords can be built with the same intervals.
Chords with different root tones based on a Major scale
So far, we have looked at how chords with the same root can be constructed from a scale. We will now expand into chords with different root tones, but still only use one scale for the building blocks.
Again, we use the C Major as an example and these are several triads (3-note chords) built on the scale:
All of these uses tones of the C Scale (C, D, E, F, G, A, B):
C: C, E, G
Dm: D, F, A
Em: E, G, B
F: F, A, C
G: G, B, D
Am: A, C, E
Bdim: B, D, F
All chords besides Bdim are either Major or Minor triads, with the intervals already mentioned above. For Bdim the intervals are: root (B), minor third (D), diminished fifth (F).
See harmonizing major scales into chords.
Chords with different root tones based on a Minor scale
Finally, we shall look at the same thing with chords based on the Minor Scale.
Here, we use the D Minor as an example and these are several triads built on the scale:
See also harmonizing minor scales into chords.
To summarize: a large proportion of chords are based on the Major and Minor scales. The chords mentioned in the examples are only a few.