This same distinction exists for Chords as well. In other words, we can have "Diatonic" Harmony and "Non-Diatonic" (or "Chromatic") Harmony.
Whenever we start using Chords that are outside of a given Key, it is generally referred to as a "Key Change". There are two types of Key Changes:
1. Tonicization
and
2. Modulation
• Tonicization (also sometimes known as "Modal Interchange" or "Mode Mixture") - This is to use Chords from two different Keys at the same time. These are called "Borrowed Chords". [There are specific reasons as to why these types of Chord Substitutions work, and we will cover some of them in another section. It often has to do with the relationship of the Harmonic Intervals inside of the Chords.]
• Modulation - This is usually what people are referring to when they use the term "Key Change". It is when an entire piece of music seems to switch from being within one Key into another, entirely different Key.
To be a little more specific, Tonicization is to briefly dip into another Key, whereas Modulation is when an entire Chord Progression + Cadence is in another Key. [Some people use these terms a little differently, but these definitions will serve us well.]
Tonicization can be accomplished by playing a Non-Diatonic Chord between two Diatonic Chords in order to connect them together. This is called a "Passing Chord". We are simply "passing through" one Key without fully immersing ourselves within it. [This is similar to the concept of a Passing Note. However, instead of a Note within our Melody lying outside of our Chord, it is a Chord within our Chord Progression that lies outside of our Key!]
Modulation can be accomplished in two different ways...
1. "Direct" or "Abrupt" Modulation - This is to just switch directly into another Key. As its name implies, it can be quite abrupt. You can hear this type of Key Change very distinctly. [If you would like an example, the song "Love On Top" by Beyoncé is a good one. Starting at about 3 minutes into the song, everytime she repeats the lyrics it audibly moves into another Key.]
2. "Indirect" or "Telegraphing" Modulation - This is to switch into a another Key smoothly, gradually. This is often done by playing "Pivot Chords". Pivot Chords are Chords that are shared by both Keys. They help us to "pivot" from one Key into another.
Learning The Basics
A very simple way to learn how to Modulate is to look at the Chords that are shared between Relative Keys or Parallel Keys. For example, let's look at C Major...
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Key:
C Major
C [C-E-G]
Dm [D-F-A]
Em [E-G-B]
F [F-A-C]
G [G-B-D]
Am [A-C-E]
B° [B-D-F]
Relative:
A Harmonic minor
Am [A-C-E]
B° [B-D-F]
C+ [C-E-G♯]
Dm [D-F-A]
E [E-G♯-B]
F [F-A-C]
G♯° [G♯-B-D]
Parallel:
C Harmonic minor
Cm [C-E♭-G]
D° [D-F-A♭]
E♭+ [E♭-G-B]
Fm [F-A♭-C]
G [G-B-D]
A♭ [A♭-C-E♭]
B° [B-D-F]
The Key of C Major shares quite a few Chords with A Harmonic minor (namely Am, B°, Dm, and F). Any one (or more) of these will work to link together a Chord Progression in one of these Keys to a Chord Progression in the other. For example, let's take the following I → V → vi → IVChord Progression in C Major:
C [C-E-G] → G [G-B-D] → Am [A-C-E] → F [F-A-C]
Now, let's take another Chord Progression in A Harmonic minor. In this case, a i → iv → V:
Am [A-C-E] → Dm [D-F-A] → E [E-G♯-B]
We can connect them together by making the Am in the first Chord Progression into the Am within the second Chord Progression. Ignore the F and play them one after another like this:
C [C-E-G] → G [G-B-D] → Am [A-C-E] → Dm [D-F-A] → E [E-G♯-B]
This makes the Am into a Pivot Chord (shown here in magenta).
Since Dm is both the ii Chord in C Major and the iv Chord in A Harmonic minor, a person would only realize that we have changed Key when we hit that E because an E Major Chord doesn't exist within the Key of C Major! Play the first Chord Progression a couple of times to solidify that we are in the Key of C Major, and then upon playing it a third time, ignore the F and switch to the second Chord Progression to Modulate into A Harmonic minor. That's all there is to it! We just did a basic Modulation.
Although C Major and C Harmonic minor share fewer Chords (only G and B°), we can use these two Scales in the exact same way that we just used C Major and A Harmonic minor. Try it out. Come up with a couple of Chord Progressions in each Key, and then try to link them together through a Pivot Chord. Go back-and-forth between each of them.
The Next Step
With any type of Modulation, we usually consider the Interval that is formed between the Tonic of the Key that we are starting in and the Tonic of the Key that we are shifting into. This is called the "Modulation Distance". [We also keep in mind the direction that we moving (i.e.: are we Ascending or Descending?). Most Modulations are Ascending.]
In the context of The Tonal System, looking at The Circle of Fifths is helpful. Find your starting Key on it. The Keys that are next to it on the diagram are some of the most common ones to Modulate into from that Key. This would be a "short" Modulation Distance.
For example, if our Key was C Major, then the Keys around it on The Circle of Fifths would be G Major and F Major.
Let's compare these Keys to see if they have any Pivot Chords that we can use to Modulate between them...
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Key:
C Major
C [C-E-G]
Dm [D-F-A]
Em [E-G-B]
F [F-A-C]
G [G-B-D]
Am [A-C-E]
B° [B-D-F]
The Key A Fifth Away:
G Major
G [G-B-D]
Am [A-C-E]
Bm [B-D-F♯]
C [C-E-G]
D [D-F♯-A]
Em [E-G-B]
F♯° [F♯-A-C]
The Key A Fourth Away:
F Major
F [F-A-C]
Gm [G-B♭-D]
Am [A-C-E]
B♭ [B♭-D-F]
C [C-E-G]
Dm [D-F-A]
E° [E-G-B♭]
Again, we see that there are plenty of Pivot Chords that we can use to Modulate between these Keys.
However, the farther away from our starting Key that we go on The Circle of Fifths, the "longer" our Modulation Distance becomes and the fewer Pivot Chords that we can find. [Generally, the larger the Modulation Distance, the more rarely it occurs within music, especially Pop music.]
So, what do we do when there aren't any obvious Pivot Chords between the two Keys that we have chosen? We have to expand our awareness of the different types of Chords that there are and pay close attention to the Notes that Chords share. Remember, the more Notes that two Chords share, the closer that they are in Function.
It is always possible to Modulate from one Key to any other Key, so long as we can find a Chord that will take us there...even if we have to invent one! ☺
What About Melody?
We usually switch the Key of both the Melody and the Harmony during a Modulation. While we have been talking about Chord Progressions, the same principles apply to the Melodies that we play over them.
All of the Notes that two Keys share are referred to as their "Common Tones". We can use Common Tones to connect together two Melodies, much like we used Pivot Chords to link a Chord Progression in one Key to a Chord Progression within another. There is some overlap between Common Tones and Pivot Chords.
For example, let's look at the C Major Scale and the A Natural minor Scale:
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Key #1:
C Major
C
D
E
F
G
A
B
Key #2:
A Natural minor
A
B
C
D
E
F
G
We saw that C Major and A Harmonic minor had quite a few Pivot Chords. Likewise, since A Natural minor and C Major are Relatives, both Scales use the exact same Notes in a different order. Therefore, they only have Common Tones. Some pairs of Scales will have fewer Common Tones though, so we have to stay aware of them.
Transposition
Whenever we move a Note up or down by some Interval it is referred to as "Transposition". If that Interval moves the Note outside of the Key that we are in, then it is technically a Key Change.
Sometimes we may want to shift everyNote within a piece of sheet music by a certain amount (e.g.: to make it easier to sing, or so that it can be played on an instrument with a Range that is different from what the piece was written in). In other words, we can Transpose the entire piece of music from one Key to another Key if we move all of the Notes by the same amount.
An easy way to do this is to write out the Scales for each Key on top of one another, like we did with the C Major and A Natural minor above:
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The Key That I'm In:
*
*
*
*
*
*
*
The Key That I'm Transposing To:
*
*
*
*
*
*
*
*The asterisks represent the Notes of those respective Scales.
When looking at our sheet music, everytime that we come across a Note in the first row, we change it into the Note in the second row directly underneath it. If we do this for every Note, then we will have the same piece of sheet music in a different Key (excluding any Non-Diatonic Notes). This is much easier than trying to figure out the Interval that each Note is moving by counting Half-Steps, or some other labor-intensive process! [Of course, there are some computer programs for writing sheet music will do this process for you, and even instruments (like digital pianos) which will change everything that is played in one Key into another Key automatically. But knowing how to do this by hand is very useful, especially if it is only a small portion of music that you want to Transpose in order to find a good Modulation!]
We hope all of this has been helpful to you. Happy Composing!
