Dansm's Guitar Chord Theory
Fitting Other Chords Into Keys
As you learned in my page on chords in major keys, every piece of music you play is in a key. In each key there are a number of chords which fit into the scale, and that is what I am going to describe here. I discussed how major/minor/diminished chords fit into major keys and minor keys on other pages, and now I will extend that discussion to all types of chords: slash chords, suspended (sus2, sus4) chords, chords with added notes, etc. I suggest you jump back to the page on intervals frequently during these lessons to refresh your memory.
As you've learned on my previous pages, the chords in a key fit because they contain the same notes as that scale. For example, let's again take the example of the key of A major. The C# minor chord fits into the key of A because its three notes (C#-E-G#) are all contained in the A major scale. Look at this on the diagram below:

In order to determine whether a chord fits into a key, you have to determine the notes of the key and the notes in the chord. Follow the train of thought below to do this:
  1. Determine what key you are playing in.
  2. Determine the notes of that key by looking at the appropriate scale (A major scale when playing in the key of A).
  3. Determine the pattern of notes present in the type of chord you are dealing with (in the case of a major chord, that's 1-3-5).
  4. Determine the actual notes in the chord you are playing by translating the pattern identified above into real notes (C major chord: 1-3-5 of C is C-E-G).
  5. Check to see whether the notes present in the chord are present in the key.
Below I will discuss several examples of this thought process. Always remember the five steps shown above, and you will always know whether your chord is in a certain key. The thought process is even the same for minor keys, though in that case there are many more options of chords that fit and it gets a lot more complex. Here are the examples. Take time to think through them, and once you get this, you'll be a master of keys.
What happens if we try to play a C#sus4 chord in the key of A? You recall from my lessons on suspended chords that a sus4 chord contains the notes 1-4-5. If you properly calculate the intervals, the 1-4-5 of C# translate into C#-F#-G#, which are all contained in the A major scale. Therefore, C#sus4 fits into the key of A.

Now, let's consider a C#sus2 chord. You recall that a sus2 chord contains the notes 1-2-5. If you properly calculate the intervals, the 1-2-5 of C# translate into C#-D#-G#. C# and G# are contained in the A major scale, but D# is not.
As a second example I will consider seventh chords. Let's first consider an E7 chord. As you learned in my lesson on seventh chords, seventh chords contain 1-3-5-m7, which in the case of E translates as E-G#-B-D. As you can see, all these notes appear in the A major scale, so E7 fits into the key of A. Now let's consider Emaj7, which contains 1-3-5-7. For E, this translates into E-G#-B-D#. Obviously, D# is not contained in the A major scale, so Emaj7 does not fit into the key of A.

Next I will consider An A7 chord. The 1-3-5-m7 of A are A-C#-E-G. Since G is not present in the A major scale, A7 does not fit into the A major scale. But now think about Amaj7. The 1-3-5-7 of A are A-C#-E-G#. All these notes are present in the A major scale, so Amaj7 fits into the A major scale.
This concludes my pages on keys, which I hope were an extremely useful set of lessons. There is a lot of information contained in these pages, but if you sit down with them and study them you'll eventually get it, and once you do you'll feel like a new musician!
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1997 Daniel E. Smith.