Many composers throughout the world and throughout the course of history have used common tones to provide a smoother link to chords or to give a smoother modulation to their compositions.
What if however, we desire the complete opposite of this effect but still wish to link chords?
Joining Common Tones
The linking of common tones within chords is traditionally and conventionally, diatonic. That is to say, that if we were to go from a Cmajor chord to an Aminor chord, we would keep the C and E notes present and in the same voice for smooth voice leading.
If we look at the bollowing below, you'll see what I mean
Looking at these two chords (C and Am) we can see that both the 'C' and the 'E' belong to both chords. Therefore they are useful candidates that would serve well when moving from one chord to another.
Common Tone Reference
The book 'Theory of Harmony' by Arnold Schoenberg provides a useful matrix to easily reference the common tones of any scale.
Let's say we're in C. In order to find common tones, we just follow the table. If we want to find all the common notes that are in the II chord of the C major scale (D minor) we look at the 'II' on the far left and reference it's cells.
We see that we could hold common tones by using the IV, V, V, and VII chords with no problem.
Using the table,it is easy to see what chords can link to one another and greatly speeds up the process of composition. It should be noted however that this matrix is only useful for diatonic progressions.
Now then, as we have been adequately briefed on what a common tone is, let us delve further and see what can be done to get more out of this device. A common tone link provides familiarity throughout the progression, even with outside chords (Chords not belonging to that key) we can link these tones.
Although it is a relatively unused musical device, there are still some modulations/progressions which are used more frequently than others. From listening, I have heard the composer Philip Glass frequently exploit the common tones between neighbouring major and minor chords.
As an example:
If we take a Cminor chord and a Bmajor chord:
We can see that there is a common tone (although it is an enharmonic equivalent) present on the third degree of each chord. So, if we hold the Eb in C and move down to Bminor with the rest of the voices, we would have a smooth yet unconventional progression.
The reason this works so well is because of the previously mentioned 'familiar effect'. The common tone provides a nice link to the rest of the progression whilst the foreign tones provide a sense of movement.
The whole purpose of this exercise is to provide a common link to an uncommon sound. It is greatly recommended that you go for unusual and striking progressions just to familiarise yourself with its use.